Where to buy refried black beans, London?
I'm not entirely sure that I know what you mean by refried black beans - the refried beans that I have bought here are, I think, pinto beans. Larger supermarkets usually have tinned refried beans in the Mexican section. I've seen El Paso brand and some other brand as well (forget which). My two local supermarkets, Mile End Sainsbury's and Bethnal Green Tesco, both carry these, and they're not even particularly great stores, so you should probably be able to find them where you live as well.
For other Mexican ingredients, this company is a great resource here in London (although I don't think they do refried beans):
I hope that helps.