Welcome!
http://wbta.us/forums/index.php?showtopic=62
Yeah, the shunt is the most reliable, cost effective way to do it. You can get really fancy with this stuff, but it's all basically the same; a good shunt, who's output is measured in millivolt.
Good luck on the DC power supply hunt. I'll let you know if I come across something that might fit.
In the mean time, you might want to post the specs for various heaters that you're considering, so that we can determine your voltage requirements.