I'm a real black series fan, like to do customs etc, and work in Birmingham - i'd also love to know where people look for these. The city center Forbidden planet gets them in, but it gets like 2 or 3 cases of each wave only, and I have a job.. so its incredibly rare that I beat the other collectors to them. mostly its just a constant unchanging stock of peg warmers. Ive used a few for customs, but thats about it.
My local Toys R us doesn't carry the line, the Disney store doesn't carry them, neither branch of the entertainer near me has them either., and I've never seen one in a supermarket. I am only interested in classic era, so rebels, rogue one and OT - therefore the Force awakens stuff that's been appearing in my local home and bargains hasn't helped.
I do buy online, from FB, TRU and from staractionfigures.co.uk - that site sometimes gets older stock with damaged packages (I opena dn photograph so boxes arent important to me)
The main thing I have found utterly impossible to find are the target exclusives, they wont ship them to UK and recent ones havent appeared in any UK shops that I'm aware of.
I'd really appreciate hearing other UK black series collectors solutions for getting their plastic crack fix.