O'brien devices are one size fits all and go to 13+ (or at least they were) my first board came with the 03 ones:
http://www.wakeworld.com/boots/getboot2.asp?ProductID=1744 Look for an older pair of them because I think they changed them for 07. Also I should warn you these bindings are VERY hard to put on because the toe literally closes up on them (you have to see to understand what I mean) so you may have people struggling putting them on for long periods of time, they have NO heel hold down and create major pressure points on the toe area to the point where some of you guests may have trouble walking after riding with them on. (But that will only be for a few minutes)
Plusses, they fit multiple size feet, they are cheap and I may have a pair that I'd be willing to part with for cheap if you can't find them anywhere else (and my dad let's me sell them) but you shouldn't have a problem finding an old pair of them somewhere, I've see old pairs of them for sale all the time