If you want Mac games that run native on your mac, as well as what's been said try...
www.amazon.com
They have an excellent selection and send out internationally. Some of those games are so old they won't run on Leopard so be careful though.
The usual download sites:
Versiontracker.com
They have the biggest selection as they allow free listings. Hit games in the category listing.
Macupdate.com
They're the "me too" version tracker, but they owe me money... gits...
download.com
Smaller selection as they're "pay to display". Only 9 USD a month with free hosting but that norrows the selection.
Support your independent Mac games developer:
Freeverse.com the finest indie Mac games developer known to man,then Ambrosia, Pangea, and the rest.

Try Neon Tango, Freeverse's latest wonder.
The final option is Crossover. It allows Half Life 2/Steam to run fast and native on your Mac without a reboot, it's a neat trick.