Can i find food reasonably easy when traveling between Glacier National, Yellowstone, and South Dakota?
I have been researching a driving trip through South Dakota and Montana. My husband and I are going to visit Mt. Rushmore, the Badlands etc, then Yellowstone, and Glacier National Park. We will be camping and I have been wondering how easy it is to find groceries to stock up along the way, particularly outside (or inside for that matter) Yellowstone and Glacier National. We'll be starting from Ohio and have never been to any of the National Parks so I have NO IDEA. I am reasonably well-traveled but not in The States. So I'm slightly afraid of sounding horribly ignorant. A trip out west is LONG overdue (my husband has never been west of Chicago) and I've only ever taken a plane to Long Beach, California. We camp a lot here in Appalachia but never for more than a week and we plan on taking a month with this trip.
I think I'm just looking for reassurance that I'll be able to find food before entering the parks along the way (like I said, no idea). Any suggestions or experience would be appreciated. I don't have any specific routes narrowed down so my question might not be answerable.

Can't you just stop 6 hours before you hit the park at some town that gets more than the smallest point on the map? Where there are people there are grocery stores. Some of those areas are sparsely populated, but come on, they aren't hunters and gatherers any more.

most of the national parks have "gateway" towns that will have supplies for you. yellowstone has a store in the park at which you will be able to get a variety of things. there's also a number of restaurants in yellowstone if you don't feel like cooking. i haven't been to glacier, but as it's also a fairly large park, i would imagine they have similar services.

Try to get your supplies in towns before you hit the gateways. Prıces will be significantly lower and, to the extent the town is bigger, selection significantly better. Except for very small places, most towns have a grocery store or two and even the smallest places usually have a convenience store. Larger towns have supermarkets as well as other stores.
In the parks themselves, there won't be much other than convenience stores with hıgh prices. Stock up before you get to the parks.

You can pack a lot of stuff before you leave, since you're driving. Specifically, buy things that are packed light - not cans - and that will compact and not take up a lot of space. For instance, there are many powdered drinks now that are intended to mix with bottled water. A small box not much bigger than a pack of cigarettes will contain 10 packets, i.e., 10 drinks - rather than packing 10 cans of something. Buy oatmeal, nuts, dried soup, dried fruit, and put in zip-lock bags. Tuna, shrimp, chicken, beef all come in pouches now. Buy boxes of noodle/sauce or rice/sauce - and take them out of their boxes - tape the cooking instructions on the ziplock bag. You can supplement these basic groceries with what you come across - fresh fruit and vegetables, etc. - but you'll always have something in case you don't have an opportunity to shop elsewhere.

What #3 said. Yes, all the gateway towns will have groceries, but they will be pricey and the quality/freshness will probably not be so good. Bring a cooler (or buy one there) so you can shop in the larger towns. Also, I usually try (google) to figure out farmer's markets and co-ops beforehand. This is a part of the country not known for its good food, and you need strategies if you don't want to buy sad looking veggies and factory produced bread. In general, college towns and the wealthier towns (Jackson) will have better stores with higher quality foods.
Gosh, so complex #4. The OP is going car-camping, not backpacking. Whether I'm spending a weekend or a few weeks camping out of my car, I pretty much buy whatever I feel like eating. Obviously you'll have some restrictions on how much you can store in a cooler and how elaborate you can get with camp cooking, but taking noodle packets out of the box? Unless the whole family is squeezing into a Mini Cooper, there shouldn't be any need for that. And millions of Americans prove every summer that there's no limit to the number of canned, carbonated beverages you can transport into the wilderness.
OP, things are more spread-out in the west, so if you are camping out you might want to plan your major shopping trips for when you'll be passing through larger towns. There's always some sort of "last-minute" shopping options in the gateway communities near national parks. But really, its not more complex than camping anywhere else, only the distances involved might demand a little more forethought if you are camping in remote areas or want to avoid paying top-dollar for little selection inside the parks.

You can hear a huge sigh of relief as the population east of Chicago comes to realize that cities and towns west of Chicago do indeed have grocery stores.

I was going to say "please don't make fun of me" at the end of my post but i decided to give you all more credit. it appears you don't deserve the credit. I've been going through the forums gathering information and a lot of them are very condescending or downright mean. this caused me to hesitate before posting and even qualifying it by saying that i feared being completely ignorant. if it is easy to get food i knew everyone would think i was stupid. if others had run into trouble and would suggest stopping at certain points that are far away from the park then maybe it's actually a good question. i just wanted to know one way or the other.
You could have chosen to say "i'm sure you'll be fine. even the smallest communities will have something to eat even if variety isn't huge! worse comes to worse you'll have to drive to a lodge restaurant a couple of times." or "i've been out there you'll be fine!" or "never been out but i've never heard of people having problems before".
if anyone knows of any limited assortment stores in the areas around the parks that would also be great.

ok i wrote that last post about an hour ago in response to #1. that does not apply to all the other helpful answers given. i didn't see them until after i published my post. I was worried about high prices and getting stuck in expensive stores, which it looks like could happen. thanks everyone for you help. i can't wait to visit a part of the country i've never been to. we'll be moving soon and it'll nice to get an idea of that area. we also have some friends we'll be staying with so i'm sure they will be helpful also.
as for #7 that does NOT apply to you. As I said i live in Appalachia and I would have suggestions for people heading to the hills. clearly there are grocery stores. i was in the Peace Corps the last two years have lived in developing countries for over 15% of my life. I know that food exists.