While you won't get many parts at a Yamaha dealer, parts are available!
The XS500 was sold from 1975-1978 in North America, and up to 1980 in Europe.
The TX500 was make in 1973 and '74 and was replaced by the XS500. Just like the TX650 and XS650, they are essentially the same bike.
So eight years of production, total.
They are getting quite a following now, but are still reasonably priced compared to some other vintage Japanese bikes.
One good site is the Yamaha TX/XS500 forum.
www.XS500.proboards.com