If you get offered a contract and you want an agent, DON'T sign it (or verbally accept it--tell them you're going to try to get an agent to negotiate for you)... A contract in hand waiting to be signed is a great way to entice an agent. But remember you still want an agent who believes in you and your work and will be there with you for the long haul. Try to find someone compatible.
Call your number 1 agent pick, tell her/him the situation, see if they want to look at your MS and possibly offer representation.
If the phone is your last resort, you could try an e-mail first with a subject like "Contract in negotiation - seeking representation". Personally I'd try calling first, as e-mails are too easily left unopened for days, and you'll want an answer as soon as possible, right? If you have more than one agent you're interested in working with, may as well call them all and see who you like best after the wooing.
Good luck!