There is an underlying question here; are you earning a living making
short films? Are you earning any money making your short films?
An agent only earns money when they find work for their client so they
are looking for clients that make money. An agent isn't interested in
building the career of a new writer/director of short films. But that isn't
what you asked, so I'll answer:
You should seek an agent who specializes in writer/directors. Something
like The Agency or Berlin Associates. But a literary agency is also a good
place to start – IF you are open to write without being attached as director.
First you send in a letter if inquiry to see if they are looking for new clients.
If they are they will want to see samples of your work and who has hired
you and what jobs you have lined up. Remember agents negotiate contracts,
they do not find work.