Kate,
Here in the Boston area we have a classified ads publication called the
Want Advertiser (it is about the size of a magazine, comes out weekly and
is sold at convenience stores). Most cities around the country have
their own version.
..anyway, I found my NuArc 26-1K, mercury vapor exposure unit (with
vaccuum mat and integrating timer) for $350. I looked under "Printing
Equipment". You might see if you have something similar in your area.
These units are generally referred to by printers as "platemakers" or
"plateburners" .
Good luck!
David