http://www.newsgroupservers.net/
Read more here.
But basically Newsgroups are mainly broken down to:
1) GigaNews
2) Highwinds
3) News-service
4) Astraweb
Now you see a lot of other providers out there like usenet.net, supernews, easynews, newshosting... essentially they're RESELLERS.
If you categorize it further it looks like this:
1) GigaNews: Supernews, Usenet.net
2) Highwinds: Easynews, Newshosting
3) News-service: I dont use this and can't name any resellers off the top of my head
4) Astraweb
You need to figure out what's best. I've used Giganews resellers like Supernews and Usenet.net and I've also tried Easynews which uses Highwinds.
GigaNews is the best freaking back end BY FAR. They have the BESt retention out of all of them. What I mean is some shit gets lost and you get incomplete stuff frequently. This happened to me on highwinds a lot, but on Giganews? NEVER HAPPENED. Now the trick is to find something cheap and affordable. I used Usenet.net back in the day. The REAL GigaNews is a lot more $$$ but is the gold standard of the industry.
I searhced around and found Supernews. It's like $11.99 or something for 30 connections and you get SSL. Unlimited too. The downside is 800 day retention versus 1000+ for GigaNews. Big deal. 800 days = 2+ years. I can deal with that. When I started on Usenet.net in 2006 it was like 200 day retention. And even now it's up to 800+ days.
BTW regarding all the discussions about Usenet. It's obviously piracy heaven. Don't lie and say that you're using it for legal means. It's like why else would you buy a radar detector? You love driving the speed limit and knowing that cops are around just so you can wave and say "Hi Officer?" You love watching those movies get released for your newsgroup and then you go to Netflix and start streaming instead? Yeah. I'm not judging anyone, but let's be honest and all admit Newsgroups = PIRACY.