www.russiantortoise.net is a good general source. Also there is a book on russians by EJ Pirog.
The key to the diet is variety. If you are going to feed them store greens, don't just feed them green leaf lettuce, for example. Get collards, and mustard greens and endive, etc. Find some cactus pads (cut off the spines). I have not found russians to be big on eating hay, but if you chop it up finely and mix it in with the greens, they will consume some of it. And I mean fine - like confetti.
Flowers - I wouldn't get flowers from the nursery for food. They love dandelion and clover flowers, and if you can find a pesticide free hibiscus plant that would be great.
As the seasons change you'll find more diet options. Clover, dandelion, plantain (Plantago species), and more can be found in the back yard.
The only problem I find with feeding store greens is the lack of dietary fiber. Tortoises that eat nothing but greens often have runny stools. Hay can help. Also including some of the canned tortoise pellets (Pretty Pets, RepCal, Mazuri) can help increase the fiber content. When my animals are on a low fiber diet I include a few pellets a week of Pretty Pets or Mazuri.
Some chopped vegetables can be included as a small part of the diet. Carrots, squash, etc.
Be sure to include a calcium supplement. Calcium with D3 for indoor animals, and just plain calcium for outdoor animals.