Where can I buy Apple Watch OS7 ?

Are you talking about the physical Apple Watch Series 7, then from the Apple Store, cellular providers that support the Apple Watch, and some retailers that carry the Apple Watch.

If you mean the operating system watchOS 7, then Apple no longer offers that version of the operating system. Apple is currently shipping watchOS 8

You can't buy WatchOS7 separately,you have to buy a watch with it already installed,

Unless you're talking about something different.