If you really want a used one, Try the classifieds over at Team Chevelle's site. I have seen working used TH350's and 400's for as cheap as $50 on there before.... http://www.chevelles.com/ads/index.shtml
You can search existing ads, and post a wanted ad there....

All that being said- If it were me, i would just have your existing one rebuilt as everyone else has already said.

A Brand new TH350 out of Summit or Jegs runs about $800- So having yours rebuild can't be much more than $500- Plus then you'll know that it was all done correctly, and shouldn't go out again. With a used one- who knows how long it will last, even if it is working when you get it.

Just my $.02