When DD was looking for photo frames on ebay, they mostly seemed to be passport-sized (when she wanted standard print sizes)
Our local pound shop always seems to have them in just before Christmas - presumably so people can give away all those small prints that you have to buy in school photo packages to get the big ones you want. Of course, they often have 'To the best Granny in the world' or something equally nauseating cute on them, but maybe you could find a plain one.