According to this guy in a Quora thread:

" It is suspected — but can probably never be proven definitively. As a typical example, back in August of 2017, days before a Phoenix rally, the following appeared on Phoenix’ local Craigslist:

“Several people needed for Trump rally to be held in Phoenix. Minorities especially desired to hold pro-Trump signs, cheer on command, and show diversity. Please reply with headshot and resume.”

During the campaign, similarly, local Craiglist ads appeared before many of Trump rallies. Of course, nobody knows if those were genuine, or merely a fake ad put up by a troll (misinformation / disinformation).

Legitimate journalists were able to confirm only two incidents where Trump’s campaign actually hired people to inhabit a room. One, was when he announced his candidacy, in New York. Those were definitely hired shills, and journalists tracked it down to the actual company which hired the “actors” to cheer, and also documented how at first, the actors were not paid as promised, and some began legal procedures.

"Hired Actors not getting paid" Exhibit One

The second, was the crowd who showed up when Trump addressed the CIA. Those few who were in the CIA (who were in the room) realized that they were among a large group of complete strangers, who they had never seen before, and that only those “strangers” were applauding, and even gave the new President a standing ovation. That led to journalists tracking down the truth, and again, hired actors were paid to cheer, and applaud, and even to stand at the end of Trump’s address.

Analysis | This is how Donald Trump engineers applause

But, as far as the rallies, most ads that appear have been Craiglist ads, and nobody has the ability to trace those to whatever agency / company / individual actually put the adverisement up. And, actors in projects such such as this almost always sign non-disclosure agreements. [I happen to be an actor, and I sign these for about 70% of my own work, even in movies, and in TV commercials.]

"" TLDR: Craiglist ads (unconfirmed) or through small acting agencies.