Jay, Aaron Brothers made probably in the millions of that frame over the years. They made it in all standard sizes up to 24"X 48". They also used it in their wholesale and contract divisions on pre framed "Art" and prints.
The reason for this little bit of history is that since they made so darn many of them, you may be able to find a larger one in a thrift store or garage sale, and cut it down. Joining it may pose a problem for you, that's a different thread.
John