I have put together a guide to silkscreens available for you to buy here in the UK. I have based these on my own stash of screens, and the ease of working with the companies who supply them. It is not a comprehensive list by any means, but I have used and tried all of these (unless state otherwise).
I have listed the current prices & postage costs at the time I wrote this. Do shop around, there are lots of places carrying the same screens and some sites charge less than others, where possible I like to local and individual, but that is my choice.