They're an abnormal size, so I tried looking at a few places but gave up.
I chose a frame of 12.5 x19.5 inches (the measurement of the prints). They've have enough room for it to slide in (the frames will actually be very slightly larger than the measurement you enter), but it does cover a small small amount of the sides (as all frames do), but it doesn't impact the piece much.
Only issue I have is the amount of space between the backing. Right now the glass and backing aren't firmly set at the front of the frame, but I haven't really put anything there to keep them at the front yet. I imagine when I hang it up some of that may go away. I'm also going to try ordering an extra backing to use to see if that helps.
EDIT: Ha, whoops. It came with tension strips. Fits and looks great now.