I haven't set up at any swap meet or flea market, but there is a huge one at the Rose Bowl in Pasadena. I don't recall seeing baseball cards last time I went, but I probably only covered 20% of the place, it is huge.
http://fleamarket.about.com/od/finda...fleamarket.htm
And, this one is in Hollywood- it is probably my favorite:
http://melrosetradingpost.org/
Sincerely, Clayton