Well, here's a bike in Mandarin, over by Jacksonville! Simple and reliable....three speed. It doesn't have enough bling factor to attract a thief and it's $20.00 if it's still available!
http://jacksonville.craigslist.org/bik/256128462.html
Not sure of the frame size, but worth checking on.....
Otherwise, look at Goodwill and the Salvation Army. Good bikes to be had cheap that way!
At your weight, pretty much any bike will do and in flat country for a basic commuting bike, simplicity is best! You don't need a lot of gears.