There are any number of custom framing options online. Generally, Frames by Mail has a good variety of materials at a reasonable price.
One of the major concepts to understand when choosing custom mats and frames is how to calculate the mat and frame dimensions. This is also why you leave space on the outside of your paper when you prep your pieces to paint. You want a little extra in case the cuts aren’t quite right.
Generally, it’s also nice to add more space to the bottom side of the mat. Doing so adds a bit of weight to the piece and nicely showcases the artwork. This means that if you cut the mat with 2-inch margins on the left, top, and right sides, you might try a 3-inch margin on the bottom side of the mat. For smaller artwork, consider adding thicker margins for a more dramatic effect.
To determine the overall frame size, you must calculate the size of the opening for the artwork in the mat, both the width and the height. Then, add the mat opening width to the size of both the left and the right side margins to get the full frame opening width. Add the mat opening height to the top and bottom sides of the mat to get the full frame opening height. To understand how big the final piece will actually be, you then need to add the frame width to each side measurement.
Most online framing companies will walk you step-by-step through the process, but it’s important to know how to check their work to ensure that you’re ordering exactly what you need.