Image from My DIY Wedding Day
Apparently, they're not that hard to make and relatively affordable too. Check out manzanita branches on ebay or look for them at Michaels, pot them in a nice vase, find a bunch of silk orchid flowers and wire them onto the branches. Voila! You have a guest book tree!
I haven't tried it yet, so it may be a lot harder (and pricier) than it looks, but I am inspired. That's the first step!