Getting Started: Where to Order, POs, ID#, and Vendors, etc.

Hello, educators...

I am an Engineering & Design teacher at a STEM highschool in Asheville, NC.

I am working on adding an Arduino component to my curriculum. I have tinkered w an UNO and components for different projects at home and ordered direct in the store.
What I need is a source/vendor from which to order via PBS Purchase order. I need to contact them to get a tax ID# and get an account set up.

Any suggestions? Can I go Arduino store direct? or a vendor like Sparkfun?