It sounds like your plays have already been produced, which is a good thing; most publishers won't even consider a script that hasn't been produced at least once.  Samuel French and Dramatists Play Service both accept solicitations for new scripts without a literary agent if submitted properly, so it only costs the price of the paper and postage if you want to drop them a line! 

Here's submission guidelines for those two:
http://www.dramatists.com/faqsmanager/applications/faqsmanager/index.asp?ItemID=31

http://www.samuelfrench.com/store/play_submission.php

If playwrighting is something you'd like to seriously pursue as a career, you'll eventually want a literary agent, so here's a directory... I'd suggest looking in your yellow pages for agencies first, but this page does help as far as helping you identify the reputable ones:
http://www.writers.net/agents.html

In my opinion, it's best to have one of the big two (Sam French or Dramatists) publish you... for the simple reason that every theatre in the country looks to those two houses first.

Now, if you come up dry at the two big ones, don't fret!  There are lots of smaller publishing houses to try... Dramatic Publishing and Playscripts.com are two good examples, but there are lots out there.
http://www.dramaticpublishing.com/submissions.php
http://www.playscripts.com/submit

They're not the first places people look for good plays, but they're certainly not the last either.  I know we've produced pieces from both of these publishers, at least, at our theatre.

And lastly, I think if it comes down to it, you're in a great place to self-promote right here!  A few more successful productions and positive reviews couldn't possibly be a bad thing... and regional (or world) premieres always seem to do well, so you'd probably have a pretty good chance of seeing your work on several stages just by using this board as a networking tool!

Phew.  That was a pretty long-winded answer to a simple question... and as a disclaimer, I've never had a play published, but I have had a short story and some other pieces, so I'm at least vaguely familiar with the process of doing it yourself...    Don't get discouraged if you don't get a positive response immediately, just keep on writing and banging on their doors!  Good luck to you!