I have a co2doctor[0,1] kit, a 20lb capacity tank from amazon (bought new - you can get them used but then you don't know where they've been or how clean they are...) and filled it for about $30 at Carbonic Service[2] in Santa Clara.

0. http://co2doctor.com/ (site down at the time this comment was posted)

1. http://www.ebay.com/sch/co2doctor/m.html?_nkw=&_armrs=1&_ipg...

2. http://www.yelp.com/biz/carbonic-service-santa-clara