Hello Bob,
I think it is the independent framers that seem to stick to imperial sizes. Plenty of these in my area -shops and market stalls. I've had some bargains, but the quality needs watching.
I get my frames from Ultimount through a gallery which sells my photographs-they get them carriage free. Ultimount do frames for photographers in imperial sizes. I use 14x11 and 20x16. Well made, they come in superb packaging which is handy if you want to post them on.
Alan Clark