The photo mats we are talking about are white cardstock "borders" that surround your photo when it's framed. So say you have an 8 x 10" frame and a 4x6" photo... the mat would be 8x10" with a 4x6" rectangle cut out in the middle.
You can buy some that are specifically called "autograph mats" but I imagine any white or light-colored one would work. I got mine at Walmart for $2.50. They were on the same aisle as photo frames. I've seen them at Michael's too.
I found this as an example: