Here is a good link for heater core repairs:
http://www.classicheaters.com/pages/1/index.htm I took mine apart (after I talked with these folks) and actually found that I could fix the small crack myself--but it was really small and easy to reach.
Good luck. I know what it is like to drive a car without a heater--my first new car was a '64 VW beetle. My old '49 Merc had a heater that rivaled a convection oven, though.
