NEED A CUSTOM CUT MAT?
If you can't find a ready-made frame that fits your print exactly, I highly recommend finding a frame that you like and having a mat custom cut to fit it. Getting a print fully custom framed can get expensive, but just having a mat custom cut should only run you about $20.
The easiest way to have it done is to bring the print & the frame to a local art & frame shop like Michaels or AC Moore and have them cut a mat and assemble it for you. Having someone assemble it for you will be well worth it if you don't have any experience matting or framing.
A general guideline in choosing a frame is to make sure there's at least a 2 - 2.5" mat around the edge on all sides, so a frame for an 8x8" print would be a minimum of 12x12". Square prints can look good in either a square frame or a rectangular frame, it's up to you. Just remember to keep at least 2" on either side, a very thin mat usually does not look very good.
I recommend using an archival cotton rag mat that's at least 4-ply. This will keep your artwork looking good for years to come and will not damage or discolor the print over time.
Here is a video from the website 20x200 which explains the process nicely:
http://www.20x200.com/blog/2010/11/how-to-frame-your-art.html