In the US, most of this runs through a sort of four-branched system. You have your four year universities, at which mostly you have to be an admitted student to take classes, but some of them will offer some of their classes in the "continuing education" division
There are also community colleges that offer a wide variety of courses both to enrolled students and in "continuing education" divisions.
There are also "for personal enrichment" type institutions that exist to provide musical instruction and it seems (I have not looked deeply into these) that their students are more or less a "one class at a time" group, that while there are defined courses of study, they're nothing like as defined and rigid as, say, a music major in a four year university.
Finally, there are so-called "community bands" or "town bands" - most of these are open to almost anyone, but some of them do have more stringent audition requirements. Most of these are focused on traditional wind-band/concert-band repertoire, but many have a jazz-band division.
Outside these four layers are the high end professional conservatories like Juilliard or Curtis.
Also outside these four layers are the many private entities - rehearsal bands, jam sessions, GB bands, etc., which you've basically got to know someone to find out about them.
As far as I know there's not really a "clearinghouse" in the US that would cover the four layers I'm talking about. You more or less have to go institution by institution in your area and do your research. My experience has been that if you get into something that's ongoing and populated by mostly adult musicians, you'll end up playing next to a bunch of band directors who use the ensemble to keep their hand in; and both your fellow musicians and the director will recognize someone who can really play, and they'll start referring you for other gigs and groups - the old word of mouth process.
And the US being such a widespread place, with so many variations in culture and density and history, the opportunities are wildly different in each kind of place, for a newcomer. So, for example, I currently split my time three ways (don't ask!) between the large Southern city where I grew up (but I lived out of state for almost 20 years, so I'm essentially coming back in as a newcomer), New England, and a rural southern district (largest town in convenient driving distance is 12,000 people, a small city of 150,000 or so is 45 minutes away from my house).
The landscape for amateur but serious musicians who want to play, especially jazz and related musics, is very different amongst these three areas.
When I came out of college and moved back to my hometown (a city of over a million at that time, now over 8 million), that was way pre-Internet, and I scuffled around but found a local community college that was opening an evening rehearsal jazz band. I got in on the ground floor there. Again it was like 30% band directors and pretty soon they started sending work my way, and that led to any number of professional jobs, both as a sub and then as a full member of bands. (I also started a rather successful rock and roll cover band with a former high school classmate, and that also spun off multiple opportunities).
When I moved to New England in my mid forties, I typed "community band" into a search and I got a dozen or so within reasonable commute distance - most of them without a jazz auxiliary, but one with - I joined that group, both the concert and jazz bands, and since I can read fluently and improvise fluently, both of those directors started sending work my way.
Now that I've moved to a rural district, I've been scuffling again - there IS a community concert band, but it's 45 minutes away, and the local community college (it's somewhat unusual to have such a comprehensive comm coll in a town of 12,000) just happens to have an active jazz ensemble. Nowadays, as a late-stage professional, I can take off an hour early twice a week to go to those rehearsals.
My point in the excessive biographical information is that in the US it's a patch-it-together system. You've got to be somewhat assertive in the search and in the self-promotion.
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