There are many sources of general maps of Russia available, but here we mention only true topographic maps suitable for orienteering. The absolute best are Soviet military maps printed in scales 1:500,000; 1:200,000, 1:100,000 and 1:50,000 (1:200K and 1:100K are most common). They are extremely precise (being made for gunnery) but you have to be able to deal with Cyrillic script to read them. Two large sources of Soviet military maps are:
UC Berkeley Library has many of them online, but some are only available from within university campus network. There are many more paper maps in their off-line collection.
Czech map server maintained by an enthusiast, has an invaluable collection of maps, many of which are not found anywhere else. The collection is updated from time to time by various contributors from all around the world.
Note that all military maps in scales 1:200,000 and better are still stamped "Classified" or "Secret" in Russia and you should NOT wave them in front of the officials. Several web sites in Russia hosting such map collections were closed recently by the FSB (formerly KGB) - quite stupid and useless action nowadays.