We use two AirSep Newlife concentrators when teaching our classes. With both of them running, we can still have a normal conversation without raising our voices. They aren't much louder than your refrigerator when running.
I think most everyone uses rebuilt or reconditioned concentrators. When they are new, they are very expensive. Once the medical industry gets their use out of them (which usually isn't very long) they are no longer "safe" for breathing. They become very cheap then. But, they are still perfectly fine for running a torch. Both of ours are rebuilt, and we have been using them for two classes a month for almost a year.
We bought ours off of Ebay, but I would check at
http://www.oxygenplusmedical.com/ and
http://www.suncoastbeads.com/ as well.