Currently I'm using NBA.com. See https://github.com/swar/nba_api for more details on that (I used it to create my own Rust client).

So far I'm trying to exclusively rely on cdn.nba.com endpoints. There are repos with historical PBP data too if you want to avoid hitting their API. Eg: https://github.com/ramirobentes/nba_pbp_data

If this project becomes more successful then I would look into paying for a real data provider like https://sportradar.com/

I would like to know this too - I have historically used ESPN data, but have issues with it here and there.