I consider it a bit of a scandal that paper, mount and frame sizes seem to have no respect for the fact that the massive majority of photos are in 2:3 ratio :-(
Anyway, I usually print 306 x 459 on A3+ so the border looks acceptably uniform unmounted, and it also works mounted on A2. A2 foamcore is available at a good price from
http://www.cartonmousse.net/(I think they are actually in the Netherlands, with various language specific web-sites. Delivery went smoothly and was surprisingly cheap for 20 x A2 and 20x A3.)
However, getting back to your question, the two feasible standard frame sizes are 50x70 and 60x80, so I would say you only have one place to go if 50x70 is too tight...
PS Just went to the Ikea site: wow, prices are good, 9€ for a 50x70 frame, 7€ for a frameless glass-and-clips in the same size (a "sous-verre" in French, is there a name for these in English?). Interesting they don't seem to have the standard 60x80, but they have the Marietorp in 61x91 for 17€.
I suspect that might be the frame used to hang a (rather bad) photo in the corridor at work: worth noting that it uses a rather thin and flexible acrylic which might not hold up to much cleaning.