Unless you happen to come across a boneyard that has some old school buses in it I think you will find it difficult to find used school bus body parts. There just isn't any market for used school bus parts. Most of the OEM school bus body companies made a lot of the parts and pieces that went into their buses. Even in a production line not everything will be the same.
Even still today, something as simple as a window is not as simple as it sounds. 9" or 12" opening, single or double hung sash, or width, or emergency push out are just a few of the permutations of windows.
Depending upon how many parts and pieces you need it may actually end up being less expensive to purchase another bus to use for parts.