I don't often recommend other web sites, but you've got to check this one out!!! Gems hidden in piles of junk! maybe, but I think I've just found an adjustable slit mechanism for $15!!!!
thing is when they have something really hot it vanishes in a day or so so you need to keep checking the site to see if anything interesting has popped up
bought first surface reflective mirrors, objective for a 70 mm refractor and a 3 inch reflector mirror as well as other odds and ends -eg focusers, eyepieces etc