I had thought of a bum bag and couldn't find one anywhere. My local PAH did not have any, nor offered to get any either. However, in the centre of Peterborough I found a store called, "Bags". It stocked everything from very expensive leather Gladstones to naff nylon bum bags for £3.49 - which are just perfect for dog walking! Not sure if this is a nationwide store and I felt quite mean only spending less than £5 with them, but immensely pleased to have found the perfect dog walking bum bag. Great for training too, with two little pockets for "treats".
Failing a nationwide "Bags" store, my next place would have been Boots travel section, where they sell blow up pillows etc for flights.
Good luck!
Potter