If you're ever up by the airport, there's a concession-stand supply place on North Lindbergh that will refill - C.R. Frank. They have been known to get cranky if you ask for a refill after 3:00 PM, though.
Ozarc Gas in Arnold will swap your tank out for you. I haven't been there in a while, but I think they were competitive with C.R. Frank.
CeeKay, on Manchester in the city, has been used by a number of people I know, but I have no experience with them personally.
There's a paintball place on South Lindbergh that I would use for refilling a 5# tank, but their prices on a 10# refill are way high.