I am completely ganking a great idea from a comment left by someone on the Brides Guide (Martha Stewart).
Instead of a guestbook, use an old typewriter so guests can leave you personalized messages! Better yet, use tea-stained or (faux) antique paper to make the pages even more authentic. Afterwards, you can paste the pages opposite pictures of the people who wrote them in a memory book or have the pages bound into keepsake books to send.
You can find vintage typewriters for a decent price at local thrift stores. Make sure to try all of the keys to see if they work. Chances are you'll have to have them cleaned and oiled, however if you are adventurous you can try this yourself. For comprehensive instructions on how to do this, visit this site- http://site.xavier.edu/polt/typewriters/tw-restoration.html.
To buy online, try ebay for the most affordable choices. Here are some searches I have set up for you to make it easier:
Antique typewriters (usually pre-WWII)
Vintage typewriters (usually 1920s and newer)
Another Vintage typewriter search