I checked around when I was thinking of buying a powertank. The only place that fills them in Utah County is Humphreys Welding in American Fork, but they only do it one day a week or something like that. All of the other places send the tanks to SLC to have them filled. Seemed like too much hassle to me to have to drop your tank off for possibly a week, and trust them that they wouldn't lose it or bash the hell out of it.
When I checked it was wound $15-$18 for filling a 10 lb. tank.
Hope that helps.
Did you buy or build your setup?