Where can I buy/download Commodore-64 games to be run by a Commodore-64 emulator on my PC?
Ebay is always a good place to buy the old cartridges.
And, a simple Google search will point you in the right direction for downloading ROMs.
Keep in mind that downloading ROMs is legally dubious, even if you have a hard-copy of the game. However, this falls under the US definition of Fair Use.