Weddingbee boards

So I saw this idea that I love for a non-traditional guest book...to have your guests write wishes, wit and wisdom on river rocks with a permanent marker that can then be used for a vase filler or garden decoration.  the only problem is I have checked high an low at the craft stores for stones which would work and have come up with nothing!

Has anyone done this at their wedding or know a great place to find stones which are rather flat and easy to write on...besides my backyard?!?!

Thanks!

@karitter12:  Not sure if you are still looking, but I found mine at a local landscape rock place. I called a few places first to make sure you can buy them by the pound until I found one that did. Some places only sell by the truck load or ton. I got the Mexican Beech Pebble (some call them skipper rocks I believe). Hope this helps!