If you can then check out all the vintage shops in london esp. in camden. But if not then www.sheplers.com has a great selection but thats an american site so the shipping could be a bit pricey and of course have a look on eBay.
they never really went out of fashion it was just that there was a western trend at the time. i think they can look pretty cool with the right outfit. im not into going all cowboy with an outfit but jeans tucked in cowboy boots with a plain vest top would look fab.