Letterboxd, small indie theaters, several universities near me have indie film series open to the public.
Film festivals. We'll plan date nights or weekend trips around indie film festivals within a few hours drive. Once you start going to those, you start meeting people and getting on mailing lists and before you know it you will want fewer recommendations.
I keep an eye on what my local Indy/art house theaters have coming up, it doesn't catch everything by a longshot but it usually does a pretty good job of catching some of the highlights.
I am lucky enough to have a good handful of those theaters reasonably close to me, and to some extent they seem to coordinate with each other so they're not all showing the exact same movies.
Even if I don't get a chance to see the movies in those theaters, it at least gives me an idea of what's out or coming out soon.
Also support your indy theaters in general. The ones near me are generally a bit cheaper than other theaters, and while the seats may not have the latest reclining gimmick or whatever that the major chains are rolling out, the floors aren't sticky, the audience stays off their phones, the staff actually gives a fuck, and often the buildings are just a lot more ornate and interesting than modern theaters. Just an all-around better experience.
If you live in or near a larger city, there's likely one around somewhere, there's also some smaller chains that show indie movies, Landmark theaters come to mind.
Even if there aren't any near you, nothing stopping you from checking out what theaters in other areas are showing to get an idea what's out there.
Also once upon a time I stumbled on a Google calendar that had all of the movie release dates in the US (assuming you're in the US of course) that included a lot of Indie films, but the link I had for that is long-since broken, but that may still be out there somewhere in the Internet for anyone who cares enough to search it out.