I'd go no further than your local welding shop. They'll tell you if they can get parts for it. For what it's worth, Miller Plasma is rebranded Hypertherm - you could call either one and see if they can support it.
Mac has gone down the tube over the last few years and I'd be weary of buying any welding machine or plasma from them. To be fair, I'd also stay away from Snap-On branded stuff in the welder/plasma type of tools.
I prefer Thermal Dynamics, but Hypertherm is good. Each makes stuff for Miller and Lincoln and any of the above 4 brands is the way to go.
For $500 you could get a new cheap Chinese communist unit and at least have some support for it. I don't care for them, but many users of this board say that they have luck with one.
$500 is not a great deal if you can't get a tip when you need one. THe Co2 thing sounds like PITA too.