Inter-city Bus Transport in France

I couldn"t find anything so complete in english, but it should be pretty straitghtforward in terms of destinations..
http://www.transbus.org/index.html
Long distance bus travel started just last yeat and things are changing very fast. Local and "départemental" busses have to be checked carefully too.

Thanks! This site seems to mostly cover bus options within each city, but i'll have to look more into it!

$50 Cdn is 34€ at current exchange rates. Either reduce your trip by half or save more before you come.

Bus travel is extremely, extremely limited in France; what there is, is mostly for schoolchildren and workers. There is hardly any bus travel geared to tourists. I suggest you abandon the idea of using buses in France, except on certain routes like along the Med from Nice east, and instead learn how to travel cheaply by train, for which you can educate yourself on www.seat61.com.

long-distance busses in France are an evolving market. starting point would be ouibus.com and flixbus.com, maybe megabus.com, too. be aware that for the really low prices you need to book well in advance. alternatively, you might want to look into blablacar.fr -a ride-sharing agency- as well, however be aware that they don't have fixed schedules, so it can happen to get stuck. don't forget that trains -again booked well ahead- will be almost as cheap as the bus yet much quicker, ouigo and idtgv are two subsidiaries by SNCF that will offer especially low prices. downside: lots of smallprint, limited network.

this said, I concur with bjd: even in case you manage to do a lot of couchsurfing etc., 35 Euro/day is really, really low, I wonder whether there'll be a point at which you'd prefer to have traveled less in order to have a higher allowance/day. there are a couple of odd destinations on your itinerary that don't get mentioned on here very often, nice for a change.

Megabus ( Freu.Megabus.com) serves 26 French cities including most major gateway cities: Paris, LYON, Tours for the Chateau country, REIMS for the Champagne country, Strasbourg for Alsace-Lorraine, Brive for the Dordogne region- pretty much a nice selection of cities a traveler would find interesting.

Unfortunately, the only two on your list Megabus goes to are Paris and Limoges.
You could plan your itinerary by traveling long distances on Megabus and buying train tix to some of your choices.

And FlixBus goes to Caen, Dieppe, Limoges and Nimes. You will need to research prices on FlixBus.

Couchsurfing seems to be the plan for so many people but I wonder if you know that it offers limited choices and possibilities. Many people who were very supportive burned out when so many people expected lodging and gave little in return and had never hosted themselves. If you fall in the category of non-hosts, be prepared.
Also, on the budget you've mentioned it doesn't look as if you'd have the finances to offer some sort of 'thank you' gift to your hos, which is sort of expected unless you have a skill or talent to exchange for your stay.

I would also point out that among those on this forum, those who have used Airbnb have said their hosts (and they were willing to pay!) sometimes cancelled on them for a variety of reasons. The same thing happens with Couchsurfing too, I'm sure, so it's nothing to count on.

If you will travel by bus or coach in France have a look here: http://europebuspass.com/bus-coach-travel/bus-coach-travel-france/