I know the mkII's go for less than $1500 all day. The mkI's are much harder to come across, but I would assume you could get them for as much if not less.
It sucks that your uncle left you hanging like that.

Most of supra forums have some mkI owners on there. Here are the ones that I know of:
http://forums.celicasupra.com/
http://www.supraforums.com/forum/
http://www.toyota-supra.info/forums/