Do you want nice ones or super disposable ones? John Lewis carries loads of outdoor safe plates, glasses, etc. And Sainsburys have had a few things in their outdoorsy/BBQ/summer stuff aisle. Check the yellow pages for party stores - you might be surprised. I've got one across from work, one in the next village over from home, and one on my route into work.
Modified to add: sometimes larger card shops have party supply sections too.