You should be able to buy this fish from any good tropical fish dealer, but don't just buy one - they'll be much happier, and live much longer if you keep them in a small group; four of them will be about right for a small to medium-sized tank.
You probably realise that there are several different types of algae, and not all algae eaters will eat all types. Otocinclus (commonly called the Dwarf suckermouth catfish) is about the only fish that will eat all types of algae.
When you are considering a purchase try to find out how long the fish have been in the shop. These fish are prone to die quite suddenly, and for no apparent reason; if they've been in the shop holding tank for a couple of weeks they are likely to be OK, but take a good look at them - you're looking for fish that appear to be fairly fat around the middle. You'll need to keep the tank water in first class condition if you want them to survive.
One word of caution, Try to avoid the common Plec. It grows VERY quickly and you can find yourself having to return them to the shop. They are a purple colour so easily identified. Go for a sail fin (green) if you can find one, they don't grow quite as quickly.
Before my leg went, I kept a tropical tank and Plec's are ideal for cleaning your algae problem. Remember that they won't eat hair algae (grass like and grows in clumps on the rocks)
You should pay no more than £3 or £4 pounds for a small to medium sized specimen. Don't be fooled if you are offered a small one, they are avid grazers and will quickly take care of your problem.
Hair algae requires treatment with a chemical solution. It's a fascinating hobby and one I really enjoyed.
My next door neigbout took over my tropical tank in addition to a 600 Litre tropical reef tank. I will upload some pictures if I can squeeze some more room out of my Flickr account!