Add thrift stores and craigslist to the recommendations above. I've seen some decent steel frame bikes at Goodwill and the like for under $30. They usually do need TLC but it's a good way to learn about fixing a bike. Some of us have even found abandoned bikes in popular illegal dumping grounds and next to dumpsters/garbage pick-up area on garbage day. Good luck!
Here are a few Bay Area claigslist listings I found by limiting max price to $100:
Step-thru frame, looks like its in nice condition.
http://sfbay.craigslist.org/eby/bik/875140081.html
Hardtail MTBs could easily be converted to wet-weather-stomper, commuter, or grocery-hauler bike!
http://sfbay.craigslist.org/nby/bik/875164656.html
Needs a lot of elbow grease, but has a lot of potential. I can't tell what brand it is from the pic but might be a classic.
http://sfbay.craigslist.org/pen/bik/875375355.html
Everything in moderation, including moderation.
2007 Rodriguez Adventure/B72
2009 Masi Soulville Mixte/B18
1997 Trek 820 Step-thru Xtracycle/B17