To buy a Stark domain through the Starknet Snap, you can use the buy_domain script from this repository. It will mint an identity, buy the domain and set it as main domain. To check available domains you can go on https://goerli.app.starknet.id.
Create a .env file following .env.example.
In buy_domain, update your domain name (line 58) and your identity number (line 59).