I really don't have to have them, but it's become a personal challenge. In Canada and the US, I bought them at Home Depot or Canadian Tire, cheaply, in large rolls. Here, I can't find them at all so far. (They're useful for storage of non-trash goods on a temporary basis. Good, therefore, for the casual moving of soft goods.)
Edited to add: Lakeland and Robert Dyas didn't have them either, but Waitrose came through for me! 2.99 for a pack of 10 transparent draw-string bags sold as "Storage Bags". nicolai also offered some good, if more expensive, alternatives, should any of you in the UK ever be truly desperate for such things.