Im having a lot of trouble buying the hops I want (specifically Horizon)
How do you find good hop stockists? Im from the uk.
There are a number of homebrew shops online in the UK, it's just a case of trawling all their websites and seeing what's available. I have heard of people ordering hops from retailers in Australia and having them sent through the post, which might be your only option if none of the UK shops have what you want.
Here's some links to a few UK online homebrew shops:
If you can't find any, you could always substitute Magnum, according to http://byo.com/resources/hops
The Home Brew Shop in the UK has a decent range of hops:
Although I don't know if they have Horizon specifically. If not you can always ask them, at least they are always quite to respond to my questions.
Failing that, since Horizon is a U.S. hop, try a U.S. shop. If they come vacuum packed they'll be fine mailed.