Just like creating the image itself, I've always been taught that how you display your image needs to have the same amount of forethought. Its good that you are figuring that out.
Squares are very stable and can lend a sense of stillness and stasis to the subject matter. If that is an important factor in your work you may want a square frame. I have seen it become popular with a lot of shows I've gone to where the photographs are framed to edge. Not in line with my personal taste, but if that is what the image dictates, go for it. Adding weight around an image isolates it, and keeps the viewer from being distracted by things competing for the viewer's eye around the image. It also presents the image as a whole, meaning that there isnt anything that the artist "covered up" by the frame, if they decided to frame to edge.
Lots of things to think about. I'm not sure about the shipping, but I have used an art supply company, dick blick (dickblick.com) to order custom frames of all sizes. They are relatively inexpensive and have good customer service.
I hope that your show goes well.