Hi -- I picked up the letter stamps from the craft store -- they were located near the cement and brick kits (this is also where i found the little bag of concrete and the concrete dye). If i recall, they were under $5. Go to this site http://www.butlerscraftsupplies.com/Mosaic%20Tools%20and%20Supplies.htm
and scroll down to the middle of the page under the heading "stone stamps." I used the traditional letters and numbers, but i see they have several other types of letters and some cool designs. A note about the stamps, they are kinda cheap feeling plastic, but they did the job.
As for the mold, i used a foil loaf pan from the grocery store. The pan is quite flexible, so i didn't have any problems removing the stones. I'm not sure if they would be harder to remove from an actual stone mold.
The project took a few hours, as you have to mix the concrete, add they dye, pour the mixture into the mold and then wait a bit (it will say on the concrete mix). Once the concrete has firmed up a bit, but is still soft, that's when you stamp them. And if you make a mistake, just smooth out the surface of the concrete (it will be thick like brownie batter) and just stamp again.
hope that helps
Wow, those look great! I've been tempted by the kits but you're right. They're pricey! Could you tell me more about your letter stamps? Where did you get them and are they expensive? Were the bricks difficult to remove from your mold? Sorry for all the questions, but you've inspired me. I want to make some of those!