Okay. I know you're in Britain, so I don't know if they have these, there, buuut.
Secondhand shops (used clothing) are great. Clothes for cheap. 😉 You can usually find leather or faux-leather jackets there for really good prices.
Jeans, leather pants, mini-skirts are also great. For a Tara costume, or even Willow, long skirts are great. Sweaters and peasant-y tops and screen-printed tees are great. Fuzzy sweaters are good, too. Tara's colours would be more subdued and pastel, Willow wears bolder colours... red, blue, purple, etc.
Boots are good for all the chicks.
For a Giles costume, a suit... preferably tweed.
For Faith and Buffy... more form-fitting tops, maybe camisoles or tank-tops, etc. Buffy tends to dress more feminine than Faith, so more flowy-ness with Buffy tops. 😉
Dawn would dress younger, keeping teenager in mind. Knowing Buffy being over-protective in her way, a conservative teenager. Jeans, a t-shirt, an argyle sweater like she wore once.
Xander wears a lot of t-shirts and over them a plaid flannel shirt un-buttoned. Again with the jeans.
Anya dresses more provocatively. Not slutty, but sexy. She'd wear sparkly shirts, or simple shirts that have ruffles, but are well-fitted, etc.
Spike would wear leather jacket, dark t-shirt, red or blue dress shirt over it.
Angel would wear a leather jacket and a dark long-sleeved sweater type shirt. His hair would be stuck up with gel.
How's that for a start?