Hi Kevin,
I think this sounds like a good idea and you should present these items in a special category exactly promoting it with "we go hunting for it for you and find it ... if not fount here...". I think many people are looking for rare replacement parts and even could contribute finding one if someone is looking for it. This will be good for the User Experience and engage users to have a look more often on the page.
May be post a link of your site what it looks like right now and people may even suggest here how to include these specialities ... so you can test the UX right here