I come accross this often. The problem being storage space.
"Photography shops"? What were they?
Where I live, you will have large shops selling cameras, printers, printer-paper, etc (think B&H). They typically don't sell frames. I'd suggest that they might sell plain frames that fit standard printer(paper) sizes at reasonable prices if it was available. "Black simple frame, glass, A3+, pre-cut portrait/landscape white passpartout, " $50 seems about right, perhaps $100 if the finish was good.
You will also have "frame stores". These seems to not care much about the photography market, but they will happily (custom) frame a photographic image at significant cost (I paid $300 a piece). I don't think that they would understand the significance of "A3+" and 3:2 aspect ratio.
I looked at ordering custom complete frames from americanframe.com ($64 a piece), but shipping to Europe doubled that amount, and local taxes are 25% + customs handling so it was still more than $150 a piece. So much for protectionism...