Craig Shier presented a program on matting prints at the meeting of July 19.
He included information on selecting a mat, sizing and cutting.
The presentation charts are available in .pdf form at the following link: JCC Matting
The charts include a number of links to websites where one can purchase mats and tools. The links are only intended as examples of reliable sources. There may be others.
If you have a good source or recommendation, please reply to this post.