As a general rule, they didn't. The models found them. They were paid.
When Bob Mizer started Athletic Model Guild in LA after WWII, a large number of his models were service men picking up a little extra cash. When the Supreme Court relaxed the laws on pornography in the late 60's and early 70's, male photographers dumped the posing strap and went to full nudity. Mizer claimed he missed the strap days, as most of the service men wouldn't do full nudity. Also, some of his most popular models didn't...er..."measure up" in the new environment of full nudity.
To replace the service men, Mizer was forced to rely much more on Hollywood "hustlers". Mizer periodically sent out brochures with pics of his models, kinda like a naked male Sears or Montgomery Ward catalog; customers could mail order pictures and films. He also held regular private showings of his films with both customers and models in attendance. Models and customers would often leave together. Mizer never objected. As I said, the models were paid.
Don Whitman also started Western Photography Guild in Denver after World War II. His studio was on a smaller, more intimate scale. Although Whitman and Mizer had a friendly though geographically distant relationship, Whitman took greater care and focused on composition, often shooting in natural settings; he took great pride in the "artistic" quality of his photography. He also paid his models.