One of the shops in Denmark Street (which, for you US users, is London’s answer to NYC’s 48th st.) has a large range of vintage Electro Harmonix pedals in stock. Can’t remember the name, but if you go down there, you will see them in the window.
You could also try eBay. I’ve recently bought a Clone Theory from someone in the US. Once you’ve added shipping and a 110v transformer, the cost starts to creep up though…
If you wish to go the internet route I would say you should try out what is at Harmony Central. I was able to purchase a Blue Box from there (which took me a while to find) and it seems they have quite a good range of pedals. Just search for the pedals or you can put up a Wanted add on the site. Also for all you fender guitar lovers you could check out FenderForum.com and that also has a wanted/for sale forum and the people are really friendly there. Hope that helps.