1

Dry mashed potato is available in Russia, but Its not something I've even looked for to be honest. You should be able to find foods such as dried pasta and noodles without any problem though.

2

How long are you going for?
Anyways, i'm not exactly a major trekker, but the last one I did was for around 8 days on the east side of Lake Baikal leaving Ust Barguzin...many years ago.
For this trip we took rice in 2 ltr bottles, our main meal..we used to just boil up rice with some tinned meat added and some stock...also some pasta/macaroni thown in, to make a stew. Can't remember the russian name for it.
Pasta, Rice, stock, tinned meats, condensed milk, break loafs, soup, tea/coffee/sugar/jam.
Can't remember any other foods outside of those really, not sure what veg we had. - it all tasted EXTREMELY good after being outdoors after the first 2 days,
Berries from the mountains amde us some good Kompot.You could go fishing if your route crosses along a river also...

Potatoes are a bit heavy and won't last as well as rice/pasta but depends on how long your off for.

3

We got most of these in Ust barguzin, but the coordinator had them arranged before hand, so was probably sourced from Ulan Ude where everything is available...like Irkutsk of course.