'Vintage' clothing is really big right now... You can buy authentic vintage items from online retailers and specialist vintage shops, most of which can be found in every major city in the country. The high street also offers a range of vintage-inspired new designs, which range in price and quality depending on where you shop. For the high street, I recommend H&M's enviable collection. They offer a wide-range of vintage-inspired items for a reasonable price (usually much cheaper than TopShop and Miss Selfridge).
For those authentic one-off items, however, you've usually got to be willing to loosen the purse strings a little as whether you're buying online or from a specialist independent retailer, the best items never come cheap.
If you're looking for a bargain, you're going to have to do a bit of leg work and shop around. I recommend Ebay as your starting point - you have to trawl through pages and pages of rubbish before you drop on anything interesting, but you can usually come across a decent bargain if you look hard enough! Be careful though - it's addictive and you easily end up bidding more than you intended (I do that a lot!)
Better still, if you're willing to travel about and physically sift through item after item then Vintage Fashion Fairs are your best bet - that's where those expensive independent vintage boutique owners go to get their stock so why not go directly to the source? You usually have to pay a couple of pounds to get in but the range of items and competitive prices will make this worth it, especially if you're wanting to buy a bunch of items at once. With clothing, you may be charged by weight, meaning you can pick a wide range of items for one overall, very reasonable price.
My personal favourite is the Affordable Vintage Fashion Fair, who organise events in cities across the country. They're the largest vintage fair in the UK and price check all their stalls to bring you items up to 75% less than you'll find on the high street. You can find loads more information on their website here
If you can't easily get to the city, another good option are Charity Shops. Although often full of a lot of, well, junk that other people didn't want, if you're willing to trawl from shop to shop hunting for that vintage item, you can quite often drop on a bargain that the shop had overlooked! I've lost count of the times I've come across a gorgeous vintage bag in a charity shop that only cost me a couple of pounds.