Welding company, company that service hospitals.

I use Airgas, http://www.airgas.com/?gclid=CIq85dWw88ECFcFj7AodaS8A0A

The highest cost is your regulator and the first tank but after that I my cost has been $20 a year vs $5 every month at the paintball places (which are rare and mainly open on weekend) and most importantly you get professional service.