The unlocked part is easy
The phone can be bought from ebay, which is safe they guarantee your purchase (read up on their policies) http://shop.ebay.com/?_from=R40&_trksid=p5197.m570.l1311&_nkw=unlocked+iphone+3gs&_sacat=See-All-Categories
the problematic part would be to actually find it within your price range (in good condition), the alternative would be to hack it yourself which goes entirely into another question (*hint hint) in which case you can save your self a couple of bucks. As for Russia and China I cant guarantee the jailbreak will work equally as you need the radio info and other stuff for the break to work, but assuming its the same carrier it should be fine (ie. the jail break for T-mobile and Verizon is slightly different).