I've had this problem before (I don't mean the electric one, but going without a heater) and I used one of those rubber bottles that you fill with hot water and old people use them to heat their back and what not. I had two of those and put both in my 10 gallon and they kept the temperature at around 22 degrees all through the night. I only woke up once to refill them with hot water.
Did you finally get the heater? Are the turtles okay?
