If you are purchasing software for your organization, your best choice is to go through the Fast Spring store. It supports a number of purchase models, volume discounts, and it isn't tied to your personal iTunes account. Purchases through Fast Spring are ideal for computers on protected/isolated networks because the software makes no attempt to "call home", perform automatic updates, or take any other automated actions that might give away its existence or conflict with your isolated network requirements.

Buy it through Fast Spring.

If you are purchasing software for your own computers, the best option is Apple's Mac App Store. All software updates are performed automatically via the Mac App Store, and you can install on all your own computers.

Buy it through the Mac App Store.