While there is no default way to fetch services, most OpenAPI specifications of popular third-party libraries are published online. api.guru has a great list of them.
To mock the Stripe API, for example, you can do the following.
$ mkdir -p __unmock__/stripe && cd __unmock__/stripe $ wget https://raw.githubusercontent.com/stripe/openapi/master/openapi/spec3.yaml
We recommend checking all service specifications into version control.