Larry... there are few sites that sell the chips...
http://www.hobbyroms.com/ is one. I think this is the one that a lot of guys use. I know I have it bookmarked at work.
As far as knowing what chips to replace, the best thing is to get the game manual for each game. Most of the manuals are available for free download on KLOV.com. In the manual it shows a layout for the pc board with chip locations.
As far as a site for which boards are interchangeable, I don't think there is one site that shows that. I spend some time at KLOV forums and RGVAC newsgroup for info.
If you want rom images for MAME or many other old console games, they can be downloaded at
http://romhustler.net/