if you live near leeds go to either a place in leeds market or inside the headrow shopping centre there is a little stall that sells them. 100% real hair! you usually cant tell they are fake.
My friend got great cheap ones in salon services, an when she changed her hair colour she died them to match and also got them cut to match the lairs in her hair
Rapunzels Hair Extensions are a decent place to buy Clip in Hair Extensions. They ship to the UK only. Also, places like additional lengths etc.
There are loads of places that sell these. Probs best to stick with the branded ones like Clipinz though. Also, human hair extensions are better than synthetic ones, but it depends what you are after and how much you want to pay.