Either dealer, or through Craig's List locally so you can go inspect the bike in person before buying. (May want to take along a bike mechanic.) Examples:
http://losangeles.craigslist.org/lac/mcy/1346382436.html
http://losangeles.craigslist.org/sfv/mcy/1345237485.html
http://losangeles.craigslist.org/lac/mcy/1343143697.html
http://losangeles.craigslist.org/sfv/mcy/1342259039.html
http://losangeles.craigslist.org/wst/mcy/1338369607.html
....
Lots to choose from in your price range. If you have any doubts about a particular bike pass it up and go on to the next.
http://losangeles.craigslist.org/lac/mcy/1346382436.html
http://losangeles.craigslist.org/sfv/mcy/1345237485.html
http://losangeles.craigslist.org/lac/mcy/1343143697.html
http://losangeles.craigslist.org/sfv/mcy/1342259039.html
http://losangeles.craigslist.org/wst/mcy/1338369607.html
....
Lots to choose from in your price range. If you have any doubts about a particular bike pass it up and go on to the next.