well...look for welding supply places (some fill co2 and some don't) or microbrewery places. down here, i go to the brewer's guild off of nasa road 1. it's about $2 per lb. in texas city there's calico welding supply at about $1 per lb.
but wherever you go, make sure your co2 tank is as cold as possible. put it in the freezer before you go that way you can get the most amount of c02 into your tank. for example, i took a room temperature 10lb co2 bottle to get it filled and only got around 4 lbs in it. but, when the bottle was cold, i got over 7 lbs.