email, email, email, call, call, call
Find one in a town, ask for the next, ask lots of question, etc. Find names, numbers, and institutions. That's really the only practical way to go about doing it methinks.
Also, craigslist ads in the photo sections of every big city (and small for that matter). There is not gonna be some comprehensive "list" or salvage company. Every one will be different, and it will require a good deal of luck. But luck can be coaxed out by ardent investigation.
Put on you helmet & light and go spelunking...