I'm looking for spritesheets for classic retro games, and old home computer games (c64, amiga etc.) Does anyone know of a source online?
http://spriters-resource.com/ has tons of different spritesheets from games.
Google for "STOS sprites" and "AMOS sprites".
STOS and AMOS were basic variants for the Atari ST and Amiga respectively - both had large communities and there were a lot of sprite collections available in the public domain.
Also, try Googling for "TOSEC". There are some torrents (probably illegal) that collect virtually everything ever released for the Atari ST, the Amiga etc. That includes some large collections of public domain disks, some of which will contain sprite collections. If you're only looking to collect the PD stuff, you're probably on the right side of the law (if anyone cares anyway).
The big problem then is probably converting all those ancient file formats. It may be easier to view them in an emulator and get screenshots.
This is not from old games but it sure has some sprites: http://www.reinerstilesets.de/