USG Topo maps are a good source of info. Your local library contains a world of info in old newspaper files and old city directories that list places that don't exist anymore. Check the religious section to see where the churches held outdoor rummage sales, church socials, outdoor baptisms, church picnics, etc. Look for sites of old outdoor skating rinks, race tracks, carnivals and fairs, old drive-in movies, Fourth of July celebrations, boat races, political gatherings, Fraternal Organization outdoor picnics and feeds, or any outdoor event. Check newspapers for natural catastorphies, such as storms, floods, airline crashes, train wrecks, etc. The list is endless. If you have any old abandoned raiulways in your area check for old telegraph lines and the glass insulators on the poles, also date nails in old railroad ties. All this stuff is collectable. Check the lightning rods on the roofs of old or abandoned homes. In the old days they were made of platinum. A lot of old abandoned homes are still full of stuff left behind.
We found an old abandoned home in Northern Idaho many years ago that was full of stuff. There's stuff to be found anywhere. Find some real old dumps and do some dump diving. You can find most anything in these. My buddy and me found one out in the forest one time and uncovered parts for an old 1920 something roadster that we later sold at a classic car meet and flea market for several hundred bucks.. The sites of old outdoor flea markets are a goldmine, as are old outdoor auctions. The list goes on and on.
Bill