Almost all of my watercolor artwork is painted on paper that is not frame-standard sized. Most standard frame sizes are geared toward photography (8x10", 16x20", etc.).
For years, I've used American Frame from their catalog and then online. They have a great variety of high quality frames, acrylic "glass," custom cut mats, and backerboards for very reasonable prices, shipped to your door for assembly.
All you need to start are the dimensions of your crazy piece--you can even see what the finished piece will look like in the mat and frame if you upload a digital copy of the art. Never been disappointed ever.
Especially for my 9x12" map paintings, I love the look and ease of a floating frame. A floating frame has glass as the backer, so instead of the mat framing the art, you can see through to the wall behind.
This is really advantageous odd-shaped pieces or for paper junkies like me who love to see a great deckled (torn-looking, handmade uneven) edge to a piece of paper. Umbra makes a good one, and can always find them on Amazon (11x14" fits a 9x12" artwork.)