Also, I have seen many people use variations of the Leman Russ, where would I find those rules? SOrry If I am becoming a nuissance :p
It would depend on which tank you are talking about. There are the Old Codex tanks, which are listed in the Armoured Company tank list, also found on GW's website. These are the Exterminator and Vanquisher. The same list of tanks also displays the Conqueror and Destroyer Tank Hunter.
Other tank variants had models made by Forge World, and had rules released by Forge World. If you can, find a copy of Imperial Armour. Otherwise, you can find them in Imperial Armour Volume 1.