I also work for a school where we had to build the library up slowly and surely, so I feel your pain! I don't have much advice in the way of shelves, but keep looking for businesses who are getting rid of their shelves, also check in with the public libraries as sometimes they change their shelving too.
In the way of books there used to be a place called the Book Bank and it was a non-profit that ran out of the old ATT building downtown KC. You had to make an appointment but once you got there you could take as many books as you wanted and they were all free. In fact, the woman told me over the phone one time that unless I was going to take 100 books it wasn't worth her time! ha! I haven't used them in a long time, but it might be worth checking into!
Finally, and I'm sure you've done this, but look into grant writing because you can get a whole lot of brand new shelves with some grants that are out there! Good luck...