Fear not, no need for custom framing. The 13x19 prints fit directly into 13x19 ready-made frames available at craft stores, framing shops, online (Framebridge, Frame It Easy), etc. There is usually not a large selection, but I have found that Michaels Arts & Crafts has the most choices. If you prefer framing with a mat, and also for a larger variety of ready-made frames, I suggest going up to an 18x24 frame. 13x19 for 18x24 mats can be ordered precut online or you can have one custom cut at a frame shop.
I have also made a Pinterest page with some selections: www.pinterest.com/dolamore/framing-13x19-prints/