Try: www.fabfree.com
they have 2 pages of links to free things for kids( and other categories too). On the ones that are sponsored by a big company, I'd suggest using the contact us or similar link to send an email requesting quantities, explaining what they'll be used for--I've found that most companies are happy to oblige. Ford Motors has a Blues Clues packet for younger kids about safety that is really nice.
I'm trying to remember, but I had gotten a box of bookcovers with all the cartoon characters(Kirby, Teenage Mutant Ninja Turtles etc. from( I think) Fox Kids. It had enough to give one to each child in the school.
Hope this helps.
The best place I can think of for things like that would be Oriental Trading wwww.orientaltrading.com/
Plus they have certain times that with a $60 order you get free shipping.