They make a percentage of jobs they book for you or work with you on, any agent you have to pay up front is a scam, so be careful!

Because of that it makes it more difficult to sign with one, because you have to find someone who sees something marketable in you or else they aren’t making a cent either, so I would suggest just making yourself known to some agents by emailing them. I can’t testify to whether or not they might find that annoying, but it’s the only thing I can think of! :P