Most pressure sensitive graphics (generic term for the type of product Letraset is/was) are difficult to find because the need for them has been replaced by computer graphics.
I used to use them but I found that they were intended for short duration use in production on reprographics and do not have long term stability. I recently pulled out some "camera ready art" that we produced with pressure sensative graphics and the letters were all cracking (about 20 years old and never exposed to light.)
Here is the website for Letraset USA
General catalog:
http://www.letrasetus.com/start.htm
Link to the dry transfer page:
http://www.letrasetus.com/LetrasetUS/Templates/Letraset/mainscreen.asp
There are films available that can run through a laser printer and then can be rubbed down on a smooth surface.
There are films available that can be put over laser printing and rubbed with a warm iron to make them have a "foil" appearance such as gold, silver, or other metallic colors.
We prefer to use silkscreening to print on mats. We use "sized" mats that are appropriate for french matting (usually indicated with a "V" on one brand, or in the description from another. The sizing is important so the ink doesn't "crawl".
There is a set up cost to make the screen(s) depending on color, etc. so if you are only doing one mat, it can be expensive. But, for awards and multiple mats, it is the way to go.
Also, Wizard has their Da Vinci machine that prints directly on mats. They may be able to do a short run for you.