If you need to buy a power adapter, one option is: Buy 20W USB-C Power Adapter - Apple
or headphone adapter: https://www.apple.com/shop/product/MMX62AM/A/lightning-to-35-mm-headphone-jack-adapter
Also, see the following from Buy iPhone SE - Apple
What’s in the Box
- iPhone
- USB-C to Lightning Cable
As part of our efforts to reach our environmental goals, iPhone SE does not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports.
We encourage you to re‑use your current USB‑A to Lightning cables, power adapters, and headphones which are compatible with this iPhone. But if you need any new Apple power adapters or headphones, they are available for purchase.