Get the PTA(or fund-raising committee)on the case to get some handy Parents to make them.Make a bit of a social occasion, with a barbeque or the like and you will probably be oversubscribed with wielders-of-hammers and bashers-of-six-inch nails !!!!
We made some ourselves with some decking wood, which is tanalised and so doesn't need to be treated, they were easy to make and worked out quite reasonably priced too, may be a good idea if you can find some dads who enjoy diy, maybe have a competition for the best one.........you may even get more than six!