Plasmafire's use a cold plasma method and they have two models
($1800 and $3200).
Longevity uses cold spark corona discharge and they also have
two models, the EXT-50 and the EXT-120 (I think about $1000 and
I personally own the EXT-120 and am very happy with it.
There are other medical grade units I know very little or nothing
about. For example, there is one manufactured by Synergy Systems
and sold on ebay down in the $300 to $600 range. The owner of
the company did post some information about them on this forum a
month or two ago and as far as I could tell from his detailed
description, they looked like good medical grade units.
Use the green welding tanks for oxygen. I use 20 cubic foot welding
tanks. Never have had anyone question me while getting it refilled
at a welding shop. Also, the pressurized oxygen comes from the exact
same place as medical oxygen.
If you're at all worried about little particles coming from the tank,
you can buy inline filters (I think plasmafire sells them).