I am also doing a postcard guestbook.
I had my friend find me 40 postcards when she was recently in Mexico (some are dated, lol, does noone buy postcards anymore?)
I plan to personalize them on the back with our names, wedding date, and location, but will have them scattered on the table for people to sign. I bought a nice box/suitcase for them to put it in when they are done.
Once home I will find a album for postcards to store them.