Here is a list of suppliers that I no a Habia Approved Training company use... Can't say I've ever used them myself as I have a private supplier.
Pauls Hair World
Maxstrand
I&K Hair Extensions
Hair Dreamer
Elite Hair
Foxy Hair
UK Hair Supplies
Hair Doctor UK
I'm not recommending them I am just providing you with a list you may find of interest (just for the record incase anyone feels I'm "advising" you.
A full head on average is 200 pre glued strands...
Maybe check Gumtree in your area for extensionists.
REMEMBER: When searching for hair extensions, always do your research. You will be surprised how many people out there take it on themselves to fit hair extensions who have no training or insurance. They set up a website, copy other peoples information and work and then think it is a quick and easy way to make money. If in doubt ask to see training and insurance certificates to ensure you are in safe hands.