Why not make your own out of tile? 3/4 in. Glass tile is cheap at hd. I recommend that you get the kind that is colored all the way thru, not just colored on the bottom, if it is going out side. Print the letters in the size you want from your computer (block lettering is easiest) and match the tiles to it. Mosaic on to hypertufa plaque using thinset.
Kathy

Here's a link that might be helpful.....
http://www.moldcreations.com/MONOGRAM-NUMBER-CONCRETE-MOLD-p/1237.htm