No one particular place. You can try searching on the US Copyright site to see if the transfer after the label went belly-up was recorded, but that is rare. If the label was bought out or folded into another label, that would be a good place to start. Otherwise, you'd have to hunt down the last guy running the label and see if he knows what happened, or you could try the artists themselves in case they managed to get the copyright assigned back to them.

If the new Orphan Works bill passes, if you managed to do all of the above and still couldn't find the owner, you'd be allowed to use the sample. As of now, if you used it, you're still potentially on the hook for injunctions, actual and statutory damages, and attorney fees if you get sued.