Tillerinobot has an API, which is used for maintenance, but also provides pp values for external applications.
You can find a swagger documentation and swagger UI at https://tillerino.github.io/Tillerinobot/swagger/
To get a key, log in to ppaddict, link your osu! account in the settings, and create an API key.
If you want to support the project, consider becoming a patron:
For more info check out the Donate page!