I have never been to this festival, but hopefully that will change in the next couple of years. It looks like a blast!
I found this on a website-
Bunol is a tiny town with very limited accommodation, not worth trying to get a room for the event as it mostly consists of residential housing. The best idea is to stay in Valencia located nearby, there you will find many places to stay, and also places to go out at night. Booking ahead is advisable but not essential (may take a little longer hunting down a place to stay without a booking).
Bunol is 30 miles west of Valencia, jump on a local train C3 line) early in the morning. The local train tickets can only be purchased in the morning (cheap) of the actual day so you cannot book ahead, makes it fairer for everyone. Takes about an hour and they do not have extra trains to go to Bunol, so get there early so you do not miss one train and have to wait till the next (could be a while). Trains also go from Madrid but that would take a considerable amount of time, you could also get the bus or hire a car, but the train is more user friendly and cheap.
You can find the whole story here.
It has some great info on this festival.
One thing that you may want to consider. Since Bunol is 30miles away and you will probably be miserably soaked with tomato juice and pieces, or drenched in water from the after-the-fact shower, once the festivities end. Maybe you could rent a small car that day for, oh maybe $20(?).
Then you can drive there at your liesure, park in a safe spot with backup clothes (NEVER LEAVE YOUR VALUABLES INSIDE A RENTAL CAR- NO MATTER WHAT COUNTRY!), partake in the festivities, then you can drive back at liesure as well without having to wait, then cram into a bus with 200 other soaking wet people. Your call...
Hope this info helps you. Please take some pictures and post them here!
Oh, and WELCOME TO THE BOARDS !