Bottle caps

 https://www.bottlecaps.de/index/

http://www.thebottlecapman.com/Bottle%20Caps%20Beer%20Used%20Plastic.html

http://www.crowncaps.net/collection.php?country=us&usedness=used&type=beer&info=noinfo

http://kevin.bulgrien.net/bottlecp/index.php?view=unit#Top

https://www.bottlestore.com/bottlecapcollectorresourceguide

https://www.guinnessworldrecords.com/world-records/68611-largest-collection-of-bottle-caps

https://www.reddit.com/r/BottleCapCollecting/