https://www.ppf.org/ or https://rethink.charity/fiscal-sponsorship are the most common organizations used for new EA projects. 

(I'm not a lawyer. I'm commenting based on some experience doing similar things in the U.S.)

It depends on what area you are working on. There are a variety of orgs whose mandates span large parts of EA-space. If you know what area you're working on, I suggest focusing on orgs that are closely related to that area. I think that nonprofits have to be able to show how their activities relate to their declared mandate/mission.