Where can you buy a mosin nagant?

If none of your local stores are carrying them, AIM Surplus and J&G sales probably have the best deals going on them right now. You may want to check Gun Broker and Auction Arms if you're looking for rarer/specific models, as the MN 91/30, and maybe some Finnish variants, such as the M38 or M39, will likely be the only models available at retailers.