Looking for complex API doc & mocking tool for integration

I work in a team where most of the time the development is to do API integration with service providers outside of my organization.

For each of these partners, they show us a different set of APIs and we are fitting it to our payment system. So it turns out that managing API docs and flow for each partner, and if sometimes they don't provide a test environment, we need to mock everything for our test environment. The experience is so horrible.

Is there any better way to handle this kind of multi-partner, multi-API flow Integration?