I used Valley National Gases a few years back. Haven't need a refill yet, amazingly. Though my CO2 has spent most of it's time in storage. There's one up in Hamilton on Tylersville Road, and one right in Cincinnati, not too far away from UC and such. It was $86 for a 5# CO2, filled I think. Refills there were $8-something last time I checked, but this was a few years ago.
I might have to give the Sparkling Carbonics place a check though. The 5# is the biggest I could get away with still living at the parents house. Me and a 10# CO2 tank would've been living out in the street
Especially since I finally
convinced Mom to let me setup a 20 gallon in addition to my 29.
A 5# split between a 29 and a 20 gallon shouldn't be too bad though. Honestly, driving to get a refill every 3 months wouldn't bother me, especially if the place is close.