This is an ad hoc list of things you can't buy anywhere, or it would take just too long to find.
Plywood - and also the panel boards (slats pressed between two thin layers of wood). Not made anymore, the last factory which made them is closed. (This was 2013 - in 2017 I heard that there are places where you can buy it)
Foot pumps - actually still available in chinese shops, but regular shops don't carry them, as they are now all chinese junk and become unusable after six pumpings. (2017: saw a good old one, all metal, 800 RSD)
Mercury thermometers. Allegedly forbidden because of the EU rules, can't be found anymore. Instead, you can only buy chinese junk - digital jobs which generally can't be trusted closer than 2C.
Clogs made of wood and leather. It's all plastic soles now. The last wooden ones I found were in 2013.