Old thread, but it's at the top of the search engines so I thought I'd update for any others, like me, that are struggling to find CO2 in Tampa.
A1 (Ybor) does it, but good luck getting in touch with them.
Southern and Booths only do exchanges
Tri-Tech was purchased by Airgas
The good news is that several Ace Hardware stores, including the one in Lithia, offer them for a decent price. I was quoted $18.99 for a 20LB tank. Taking my 5 over there now and will return with a price.