Go to art and craft store and look at frames and mats they have. Make your photos fit the frames. Dont do it the other way around because that is when it gets expensive.
Cheap mat and other framing materials significantly shorten the life of a print.
Mount and mat materials absorb contaminants from the air becoming more, and more acidic (acid), and begin degrading the print.
The best materials are lignin and acid free to start with. Even the best materials need to be replaced periodically to ensure the life of the print, unless print lifetime is not a concern.
Even archival, museum quality, lignin and acid-free mat and mountboard is inexpensive.