Fishtanks can be found here, takes a bit of sifting for ones in your area, and some people want silly money, but dont let them dishearten you. It sometimes takes a few days to find the perfect one for you, but patience is a virtue, or so Im told!
Ebay can also be ok, but again you have to sift through quite a bit. The benefit is that you can do a search within 25 miles of your own postcode as long as you are a member. There is also Freecycle as mentioned by Bloo.
New glass ones will be for sale in most lfs. You can have a look at sizes available on the clearseal website
here.
I would look for you but I dont know where in the UK you are? If you are in London you shouldnt have any trouble finding one. There seem to be a lot of fishkeepers in the Yorkshire area as well for some reason! I've been looking recently for a third tank and there seems to be none in Preston where I am, but loads over there!
Try checking your local paper at the back in the classifieds section or Loot (a regional classifieds paper). Loot are also online
Here but I dont know how quickly things go online. I find that I'll ring about some bargain or other from the website to be told "Sorry love, sold it last week".
Have a scout around before you decide on anything, after all, its worth holding out for the best you can get, it gives you more time to save up, and its not like the fish shop will sell out of livestock!
Jessica
Edit - Woohoo! My links worked! I'll have a look for you and see whats about. As Neal said, there are some bargains to be had. It depends how mobile you are ( or how willing your mum is to indulge your hobby by driving you round!)