Play Asia are currently selling pro controllers on offer for $28 (approx. £16.50). Also need to pay for shipping which was $23 (approx. £13.50) for me, though I don't know whether it'll be the same price for all UK addresses but seems safe to assume it will be.
Obviously it'll work out cheaper if you order more than one controller as you'll only pay for shipping once. I got two controllers for under £45 which is great considering they're £40 on amazon at the moment!!!
I think there's a limit of 2 controllers per customer but I've heard that it will let you get 2 black and 2 white controllers so it's worth a go if you're looking for more.
It's the Japanese controller but it's region free, I've ordered a couple which just arrived and they work completely fine. I also have an EU controller that I got from Amazon last year and the only difference is that the box and label on the back are in Japanese.