Apple Watch compatible with iPhone 7

None. Only units on an OS below 9.0 can be paired with it.

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If you can find a series 7 watch, it is compatible with an iPhone 6S or later.

Apple Watch Series 7 - Technical Specifications

Apple Watch Ultra, Apple Watch Series 8, and Apple Watch SE require an iPhone 8 or later with iOS 16 or later.

Apple Watch Series 8 - Technical Specifications

Not as far as I know. You will probably have to find a used/refurbished source. If you do, see the 'Before you buy a watch from a third-party' section in:

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

There are some refurbished ones currently available in the online Apple Store for the USA, but if they're on 9.0 or above, you won't be able to use them with an iPhone 7; the requirements javaliga linked to only apply to units on their original OS version.

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