There isn't a "tank" on a typical heater core. And as far as getting one from a salvage yard goes, I got mine from a salvage yard and it was ok. It would cost less than having the rad shop get one that would probably be as much or more than the aftermarket one (cuz you know he'll mark it up). You can find good cores for about $10-$25. I got the entire heater for only $60, and that was the Deluxe heater.
