How do villians get henchmen especially in Gotham City do they take out an ad or just go to a dive bar and say who wants to earn some money?

Watch The Sopranos or The Wire. Crime needs an ignored working and poor class who find it more opportune to rob businesses and guard crime lords, than to try and work a cash register or scrub toilets.

LF Thug position. $100k/y salary required. Very multitalented!

You might like Minor Threats (and From The World if Minor Threats: The Alternates, and Barfly). Minor Threats is about C-list villains in the villain neighborhood of Redport (the hood of their respective metropolis) who gather in a bar to Oceans 11 style take shit over, The Alternates is about C-list superheros in a recovery group after saving the world, and Barfly is about Shiteater who is a fly henchman of Bugman who, when Bugman was arrested, went punk, started working at a fast food place, and became a regular at that aforementioned bar, now Bugman is back and is a total dick.

Doesn't exactly answer your question but explores similar concepts of attaching reality to superherodom.

The Venture Bros has an episode where they need all new henchmen and they try to hire some gang members off the street.

In the Harley Quinn show there is an agency for the top tier ones. The regular ones you can pick up at a bar. Kite man literally bursts into bar and asks if anyone wants to do a job but it depends on the weather and everyone goes with him.

Thunderbolts and Agent X both explore this a bit, and go for the dive bar angle.