I had the same problem.
I called a lot of professional copy places and none of them could do 12x12 copies. So finally I just went to a local one and asked them to make a copy of one of my layouts. They too could not do 12x12 only 12x11 (as was the case with the other places I called). I have gotten to the point where if I know I am going to need a copy of a layout that I cut a 1 inch strip off the side of the BG paper before creating the layout this way when I do get a copy made it will include the entire layout.
As a matter of fact I had been meaning to make it a point of reducing all my layouts by one inch JIC but had strayed from that (can you believe I actually forgot! I'm an airhead)! Thanks for reminding me!
Good luck on your search...hope you're successful!