Where to buy entirely depends on what sort of educator you are. To be clear, higher education is covered by the Apple online store, as well as a number of Apple Higher Education Resellers, usually located on college and university campi. If you are a K-12 Educator, the only official option is the Apple online store.
However, problems can arise in either of these categories if you want to obtain the item immediately. Some Higher Education Resellers (primarily if the store is owned by the school itself) may refuse to sell to those who are neither a student, nor faculty at that institution. The easy answer is to find a private business that is a school's Apple Higher Education Reseller. As a private business, they're less demanding with regards to what school you belong to.
I work at an Apple Higher Education Reseller in Canada, and since we're a private business, we don't have a problem selling to K-12 educators, or students from other universities. If someone wants to give us money, and it wouldn't cause us to be in violation of our contract with Apple, then there's no reason to turn them away. Other Higher Education Resellers in the city will refuse students who don't attend their school. As such, we tend to pick up more business.