I cannot recommend Paul's Hair World, on Oldham Street, in Manchester enough.
I have worn human hair extensions for years, on special occasions mostly, though for a large part of 2009 through a bad haircut they became a daytime staple. I would consider myself a bit of an expert on hair extensions; having bought many sets and done plenty of research in my time I have not found a store as well stocked or as helpful as Paul's Hair World. So today I was on hand for my sister's right of hair passage - her first set of extensions - and the only place I was going to take her to was PHW.
Having a very multi-tonal shade of blonde makes buying hair extensions more difficult. I have always worn an 18 inch European human 6-piece set. Due to the long length and density of my hair, I can accommodate quite a lot of additional layers without looking too false. My sister has no such luxury with fine, short hair but PHW were very helpful, finding shorter lengths, thinner textures and a multi-tonal weave to match her bottle blonde highlights. The shop assistant even split a packet to only sell us 3 weaves, rather than the usual 6 a full head like mine would take.
For sheer quality of hair (my sets have each lasted me over 2 years) and variety of lengths, colours and textures Paul's Hair World is the only place to buy hair in Manchester. However, be sure to know what you want before you enter the store - staff are predisposed to a very stereotypical 'Scousewife' look and will be likely to sell you the longest, blondest/blackest, straightest and chavviest set of extensions they offer. Be sure that, if you're after a natural look, that is what you are leaving with. And do ask to try on in store to make sure hair looks thick, full and most importantly alive when worn with your own.