Any preferences or requirements? Any computational limitations (a MacBook Air might not be as powerful as an iMac depending on what kind of games you want to play).
And when getting games from outside the MAS, you can use Steam or any other online facility that sells or offers games.
Good racing games would be Asphalt 6 or (NFS) Need For Speed.
Asphalt 6 is available in the Mac App Store.
Need For Speed is not available in the Mac App Store, and I do not know of any download link.