Hi Steven!
I have to agree with Kevin and Clayton, 3/8 TK = 3/8" thick material . They are saying they made it out of 3/8" material but you can use the thickness you desire.
For backings Mirrors come in all different sizes and shapes
A good glass cutter would work for cutting , also Dremel has a Diamond Wheel That cuts glass I used it when modifying a tank for some lizards. It worked great.
For the protraits I frame I use matting board
I have used felt . and heard people using scrapbook paper to get differnet back ground design .
You can use air brush on your computer to design your own backer and print it out.
Example:
http://www.scrollsawvillage.com/gallery ... ?pos=-4511
David