There are a lot of agencies out there.
Keep in mind that the more established agencies may have deep ties to casting and to the entertainment community, but that does not mean that a smaller or newer agency lacks the ability to represent you very well.
You can try sending open cover letters to submissions for every agency in town, but I recommend that you gather plenty of information and pitch yourself to a specific, handful of agencies. Be sure that they do not already have a number of actors who are similar to you and use this as a selling feature.
Customize your email and/or letter. Refer to specific agents, personalize the note. If you saw them on a panel and were impressed, mention it. If you have a showcase or performance coming up, then offer free tickets.
Referrals are increasingly important, and some agencies will not consider talent who have not been referred. If you have two or three vouch for you, even better. These individuals can open the door for you and boost your credibility.
If you receive a flat no, then back away. You can try again after about six months if you are still in the market. If you do not have any luck with your top choices, then move on to another handful. Continue to be proactive, and you will land an agent, but it may take awhile.
Most important of all is to use this time to get stronger and stronger in you work as an actor and train, train, train.