Depends how quickly you want the games and if you want loose or boxed games.
Patience can save you a lot of money, only buy from eBay if you don't get any joy anywhere else.
Search YouTube for hidden gems videos for each system.
Metal Jesus Rocks does a lot of hidden gems videos.
Classic Game Room (Lord Karnage) has loads of reviews for popular and obscure games on loads of systems.
Game Sack has a decent channel too.
Checkout these sites as well:
http://www.racketboy.com/guide/retro-gaming-101
http://retro-sanctuary.com/index.html
Most N64 games haven't aged well, maybe try out games on your PC first before you spend any money, if you know what I mean. (E word discussion not allowed)