Well, I use Ohio Fire & Safety. 1199 Fourth St (take I-670 to US-23/Fourth St. exit, head north about 1/2 mile, it's a small building on the left).
$34.50 plus tax, takes about three days.
If you have time, a better option is to try a SCUBA/DIVE shop. Aquatic Adventures in Upper Arlington or Todd's SCUBA in Circleville. They will do the Hydro, then perform a visual (required before you fill an empty bottle and annually - cost $12) and fill it with air (typically $7) for $50. However, it takes seven to 14 business days.