Most districts lock you into specific vendors. These are usually the big four:
Of course, you can recommend a new vendor to your school, but you will need evidence to help the CFO understand the savings. This will take time and effort on your part because you will need to create a spreadsheet that lists each vendor and the costs of the items for comparison. I think you will find from your analysis, though, that the savings even out. However, I would not want to discourage you from trying because comparing prices never hurts. You may find certain vendors offer better savings on specific items or category of items. Knowledge is a good thing.
Other vendors to consider are:
I have compared prices at Utrecht and United but never found them as competitive as the big four. The bottom three vendors are those I use when I purchase supplies for my own use such as an off-site class. I have never used them for school purchases (not an approved vendor).
Do you have vendors you recommend?