What is this soffit attachment called, and where do I buy them?

L-bracket?

Soffit bracket is one name for them - look up construction details for a proprietary awning system and they usually have a soffit connection detail shown.

I doubt you can buy off the shelf anywhere except an awning company. Probably proprietary to the system pictured.

Could get custom fabricated or a large galv angle bolted to soffit with a smaller one bolted to that to connect to the awning.