This is a great head, I have 4 of them, 1 I use regularly and 3 others as spares. Often the keys get sticky and non-conductive with use, my guess is that is what happened to yours. LIkely the head is fine, just needs key cleaning. Here's documentation on it:
If you backtrack up that URL to: http://web.archive.org/web/200412041..._Downloads.htm You can find other documentation. I have a pretty full archive of the various files including the manual, I can email if need be.
Indeed the tubes are hard to find, even on ebay. It's easier/cheaper to buy complete heads as spares, since they will have the tubes already installed in most cases, and often are no more expensive than the tubes separately. The complete heads can be found pretty regularly in the $50-200 range, on ebay and elsewhere. not bad for a head that was $2500-4k when new.
This is a great head, I think the best color head available that I have ever tried. Spend the effort and get yours working, it's worth the time.