Where to buy? Looking to trade in an SE for a 15. My carrier is T-Mobile. Am I better off buying the phone from Apple or T-Mobile?

Also check which each provider offers in the way of "locked" vs. "unlocked" phones.

These days, phones sold directly by Apple in the U.S. are not locked to any carrier – except in one or two particular cases where you select a particular carrier's financing plan. Having an unlocked phone is a good thing, because it lets you switch carriers (phone companies) later, or use other companies (e)SIMs when traveling abroad.

Phones sold by a carrier may be locked to that carrier, meaning that you will not be able to get your cellular service anyplace else, unless that carrier chooses to let you go. If a carrier subsidizes a phone as part of a financing plan, you can expect the phone to be locked for at least the length of the contract – and possibly thereafter.

I'd start with your carrier. They sometimes have trade-up deals. Easy enough to check both.

That's up to you. Compare what T-Mobile will give you for your phone, vs what Apple will give you.

Here is a link to help you get started with Apple Trade In --> Apple Trade In - Apple