They often turn up on ebay and other places-- although many different brands exist, do a search for "variac" or "powerstat" as those are most common and such a search will also turn up many others. The best ones to be had will be the old US made variac (general radio) or powerstat (superior electric).....the GE "voltpak" or similar are good too. Ultimately, however, these are all pretty robust devices and all you'll need to know is the voltage and amperage rating to select one.
BUT before you go seeking one out, are you certain yours has failed? They have a fuse in them, usually, and it is easily changed. You can do a quick and easy test by unplugging it from your reactance choke and plugging in a regular incandescent lamp--does it vary the brightness?....if so, then your problem is most likely in the choke itself......if that is the case, it's probably the bridge rectifier (also an inexpensive part, let me know if that's what's up and i can walk you through it) If the variac kinda works with the lamp but has "dead spots" as you turn the dial, then it is probably the brush or the contacts that it glides across......and that is a repairable thing too!
Hopefully that helps!
