I've been lucky to find most of my replacement parts on eBay.
There are some current listing for replacement shift boots - see
http://cgi.ebay.com/ebaymotors/Toyot...20441414QQrdZ1
Occasionally a "nearly new" original MR2 leather shift knob comes up for sale on eBay. I'd keep my eyes open for them as they do get snapped up for a good price ~$60. As for your seatbelt retractor, there is a gentleman in Texas named Ed who sells used 85-89 MR2 parts on eBay. email him at
cmstexas2@yahoo.com with what you want and he will list it for you. His prices are very reasonable, the items are well packed and the quality is excellent.