Car heater not working!!?

Ok so the heater doesn't work in my car. It only works on defrost and doesn't blow out warm air at all. It is FRIGGIN freezing out now and the rides to work are terrible. With it being christmas time and having two daughters...$$$ is not an option right now. I've checked around a few mechanic shops and they all seem to say the same thing "heating core" which if it just needs to be flushed isn't very expensive..however if it needs to be replaced I'm looking at $500-$800. I was told of some other things that it could be...vacuum leak which could run $100-$150, or a door stuck(something in the dash) which again its going to run more to the $500-$800 range (for getting into the dash). I can't even try to afford this right now...and of course it's December!!! I thought of looking into one of those heaters that u plug into your car adapter? anyone had any luck with those? Also I *think* my thermostat might be messed up to...can this be the issue..or is that to simple..lol?