I am looking for stones of Apriums, Peachcots and Tri Lite as well as Canadian White Blenheim.
Have you tried the Seed Exchange Forum?
Here is a list of all exchange forums. None of them are specific to fruit.
Have you tried this member pages search? (Nothing shows up now that I see.)
One tactic I use is to look in Google Catalogs which show outdated catalogs. When I get results then I go to that company's current catalog to check if it is still available.
I've been checking the local produce market. So far the plumcots, pluots, and apriums haven't come in. Only a couple of plums. These "exotic' hybrids are fairly common in California markets when in season.
By the way, most Prunus pits don't like being dried -- except almonds. When dried they become very difficult to germinate. You should read Reychler's report.
Here is a link that might be useful: Germinating dried peach pits