Just do a Google search on "atari flashback 2 mod" (or "hack"). Then do a Google image search on "flashback 2 pcb". There is also the picture of my board from a previous post: This pinout picture helped too:
> Did you cut up an old atari and use parts or is this all from new parts? etc. >
I had an old Atari chassis lying around which I ripped the cartridge slot out of.
> I see an occasional flashback for sale cheap and this might be fun to try.
I got 2 FB2s from two different ebay sellers for $5.00 each plus shipping.