It depends on what it is you're looking for, but in general e-Bay is a good place to look for discontinued stuff. I've bought Pluto Platters on e-Bay, and they've been discontinued for about 50 years. ;)
if you are a current pdga member you could try and make a post on the "ask hero about discraft" thread under equipment. state that you are looking to buy some x-clones and see if anyone has a stockpile they are willing to part with.....worth a shot
discgolfreview is a good place to look for plastic. There are some great guys over there and a lot of plastic getting swapped around. I've grabbed some KC Pro drivers and Champ Whippets off the site and gotten rid of a lot of stuff I didn't want/need.