Russian tortoises are a true burrowing species, one of very few in the world. So the ideal substrate for the species would actually be compactable soil and several feet of it! But of course this isn't practical in captivity indoors.
A good substrate that is inexpensive and easy to maintain and replace is cypress bark mulch. It can be purchased at garden centers and is fairly cheap for a large bag. For a Russian tortoise that usually will bury itself the substrate should be kept at least 6" deep. Deeper than that if possible. Having the ability to bury itself when it wants to gives the tortoise and needed sense of security as well as at least a little bit of natural function.
Stay away from substrates that create dust when disturbed such as coconut coir, shavings or similar products.