I buy (almost) all of my stuff through The Impossible Project‘s online store. However the ProCam I bought in person at their space in New York City as they weren’t selling it online and I happened to be in town at the time.

I tend to check garage sales and antique shops whenever I can as well. My suggestion is to take a empty cartridge with a working battery with you. That way you can so you can pop it into any cameras you find and test if they can at least shoot. You can usually find a good working camera for about $40 or less. I find that almost every one I’ve seen works fine unless someone has physically damaged it. 

Specifically for the ProCam, in the UK your best bet would probably to check out Incognito in Manchester or at least call them as ask if they know somewhere: http://www.incognito-uk.co.uk/

Impossible’s dealer list might be a good place to start as well: http://shop.the-impossible-project.com/stores/europe#gb Most just sell film or small packages with 600 cameras from my experience though.

Unfortunately you may have to resort to eBay. There are always seems to be a few floating around on there from the US but I know lots of Americans are uncomfortable doing business internationally.

Good luck! I hope this helps. Thanks for writing!

-JM