Interestingly I struggle to find less popular stuff in Europe but generally the items are available in the States. I am talking about cartridges for the C64 or 5.25" disks for the Atari. Very often I am short of a cable or bit of software and I do a search and it always says available but thousands of miles in the US direction.
Posting on Yahoo groups and some of the forums did use to work but that is a while ago now. The UK is crazy bonkers in truth. The days of getting a cheap deal has all but gone. I mean I could get a 100 disks for a couple of quid. Now there are guys selling a single cover disk for a fiver. Seems to be the same people though.
You just gotta keep trying. Create a small website and create an interest for yourself and join a few groups. Broadcast your interest and you never know.