I'm late to this thread, but you can't really "hire" a literary agent. you're not meant to reach out to agents, agents are meant to reach out to you
here are a few ways to get a literary agent's attention:
*win a screenwriting contest
*enroll yourself in a renowned screenwriting program at a prestigious university (e.g. UCLA) and be referred by a professor
*be referred by an already successful screenwriter/filmmaker (this is why networking is extremely important)
*attain a low-level job at a reputable agency like CAA or William Morris, network/make connections with people above you, and show them your work when you've gained their friendship
*post your writing online and cultivate a large following
good luck.