You can purchase a heater ring from Theo
http://www.gamaelectronics.com.au/heater.html
They work on 12V and have a resistance of about 230 ohms.
You can also make one using resistors. The one I made has 48 ohms total resistance and works fine. I use a dew heater controller to adjust the temperature.
The Kendrick dew heater website lists the power ratings for al their dew heater straps and is a good starting point for sizing these things.
http://www.kendrickastro.com/astro/d...erheaters.html