WTB: mirror lens

[Aaron Reynolds]

I'm looking to fool around with a mirror lens and see if I can make it 
do the things I want it to do.

I'm not picky about focal length, speed or condition, provided it is 
inexpensive.  I'd need something in either K mount or m42.

I'm also willing to pay more for a faster one.  What I'm trying to 
determine for myself is whether or not I want to pay $200 for a 500mm 
f5.6 mirror.

Let me know what'cha got or if you have any heads-ups for me.  I've 
seen a lot of 500mm f8s on eBay in the $100 range, which is too much 
for a lens that I probably wouldn't use after testing.

-Aaron