How is the iPhone SE an iPhone X alternative? It's a small iPhone that still performs well and it's the most pocketable and easy to use with one-hand for many shoppers.
If you want to stay in the Apple ecosystem, but you need a super small phone, you should pick the iPhone SE. With a 4-inch display it's smaller than the iPhone X and it still packs in enough power for many users and a good enough camera.
Keep in mind that you may want to wait to see if Apple is going to deliver an iPhone SE 2 in the Spring with upgraded cameras, power and new screen in a small package.
You can buy the iPhone SE at Apple and from major carriers.