You don't need a tank that big - these frogs don't move very much. You should be able to order one over the internet if you can't find one locally, though there should be options in or around Bristol. If you can't find any, there are plenty of online sellers in the UK. Just make sure you know which species you're getting - Giants are Pyxicephalus adspersus. Dwarves are Pyxicephalus edulis. Some vendors either (a) mix them up by accident or (b) have Dwarves that no one wants and so they sell them as Giants, hoping that you are none the wiser.