Obviously an agent is not going to waste time on an unproven product. In fact, you may find it easier to get an agent after you have gotten a book accepted. There isn't enough money in any of that shorter writing to pay for the agent's lunches. Indeed, you might expect that the type of publication in which I am engaged here will effectively do away with the big magazines. So, unless you are famous and can command high advances or payments, you are wasting your time even thinking about getting an agent for these genre.
Agents are interested in books, plays, film and television scripts.
Generally speaking, poets need not apply. Suspect anyone who wishes to agent shorter works. They are going to bait and then switch you to some service for which you will have to pay. I can tell you how to go about trying but the process will not work until you have a track record in writing and publishing. Of course, all you have at that point is a contact.