Nzbgeek, nzbplanet, dognzb and Drunken Slug are the best indexers in my experience. Really only need 1 or 2 of these.
Providers are both trickier and simpler. most are just resellers and theres really only 2 backbones for the providers. There’s a diagram somewhere of all of the different providers and which backbone they come off of. I got NZB ninja a decade ago cuz it was the cheapest at the time but really any of them will do.