When I first started, it was mostly just asking people I knew, which can be a little uncomfortable. A couple times I placed classified ads, but mostly it was people I knew, or people who people I knew knew.
As I got more serious, I started finding models using websites like ModelMayhem. I still use that sometimes, but it doesn't see as much activity as it used to. Craigslist used to be okay for finding models, too, but since SESTA/FOSTA and the changes they made over there as a result, it's not very good for finding models now.
These days, Instagram is really the place where models and photographers connect. I follow models and photographers whose work I like, and then follow people they work with, and so on. When I find someone I'd like to work with who's local, or who's traveling to my area, I message them, and sometimes it works out.
I mostly work with established models these days. I don't do a lot of asking strangers IRL to take their clothes off (at least any more).