where do i find rocks that my wedding guests can sign? I want to do an alternative to the traditional guestbook?

Wow that sounds like an awesome Idea, well let me see what I can find for you...:)
After careful search I have been able to find some really interesting rock kits for you to take look at.
The first site I found was Premier Decorative Stones has stone kits in various shapes that has a smooth enough surface that they can be writtwn on with a paint marker. The prices vary depending on what set you are wanting the prices range from $20 to $80. Soem come in sets of 8 to 20. So I guess you would have to estimate the amount you will need for your guests to sign.
Another site that you may like also is Stone Decorative they are do it yourself naturally polished stone kits that are oerfect to write on. They come in sets of 8 and 25 with price ranges from $24-$45. They also offer write on self standing stones, they only come in sets of 4, you have to contact them if you want to order more.