Well, you could buy them.  Most software stores have a section for Mac
games, but you can usually get better prices, a better selection, and a
better return policy from mailorder sources.  Each issue of MacWorld and
MacUser contains many, many ads for mailorder companies, between which
they occasionally sandwich articles.  The phone numbers for the biggest
sources in the USA are:

        MacConnection:  1-800-800-3333    1-603-446-5555
        MacWarehouse:   1-800-255-6227    1-908-370-3801
        MacZone:        1-800-248-0800    1-206-603-2400
        Mac's Place:    1-800-249-0009    1-406-758-8000
        (formerly Computer Outfitters, 
         which was formerly Mac's Place.  Don't ask.)

They'll be happy to send you a complete catalog upon request.

There are also several mailorder sources on the WWW:
        
        Computer Express:  <http://www.cexpress.com/>
        Cyberian Outpost:  <http://www.cybout.com/>

Cyberian Outpost in particular has an excellent reputation on the
comp.sys.mac.games groups.  Both of these companies do overseas shipping.

NOTE:  Mailorder sources are NOT the most reliable source of information
about release dates for games.  Games are often advertised in their
catalogs, usually with preliminary screenshots, months before they are
actually available.  Also, the sales clerks generally have no idea when
the game will be released.  If they tell you it'll be available in "two
weeks," that could mean two weeks, two months, or a year from next Tuesday.

Local Mac User Groups are another source for games; often shareware and
freeware games are distributed on disk at meetings or are available for
downloading on their bulletin boards.  Contact your local MUG for details.

If you can't find a game anywhere else, you can always order it from
the company directly.