That's a difficult one. It's usually a matter of keeping your ear to the ground - reading lots of agents and publishers' blogs and websites, keeping an eye open on writers sites like WW, and make a note of new agents' names when they crop up.
Some might be mentioned on The Bookseller:
http://www.thebookseller.com/
<Added>As for whether you should give up submitting it, only you can answer that. Have you had any feedback from fellow writers on your opening chapters?
I find it a problem to stay motivated on a new writing project when I'm waiting to hear back from submissions so maybe it's worth putting it to one side so you can concentrate on a new novel, then pull it out afterwards and look at it with a fresh eye. Also the market is very difficult at the moment because of the credit crunch, so it might be worth waiting a few months before sending out a new batch of submissions.
- NaomiM