When should I start sending out my resume to potential employers? My instructor told us to start now, but we won't be done with the program (externship included) until the end of June.

do it now. It's April and you have 2 months. YOur instructors are right. THere is no such thing as too early. Sometimes clinics can take so many M.A externs at one time, and if you get your spot ensured early you will be more than likely to get it.
I am surprised that you have to look for your own extern.
Usually the school will have to get you one, its part of the contract.

Do it now, just put when you will be availible (right after graduation) on your coversheet. I got hiered at my externship site a month before graduation.

Oh no, I must have worded my question weird. My school does find our externship sites for us (I already have mine). I was just wondering when would be a good time to send in my resume, seeing as how I won't be done with my externship until the end of June.

Thanks for the advice =)