Where do I find it? I didn't see it in Shoprite.

I'm not sure where you live, but if you're in NJ you might want to try Kings. I find that the one near me has a lot more kosher items than Shoprite.

I live in Jersey but never heard of kings.

They're all kosher. The flavoring is diacetyl.

Spice Supreme makes a pareve imitation butter flavored Popcorn Salt.

What do dinosaurs have to do with popcorn?

Are you sure you're just not looking in the right spot? You can almost always find one by the microwave popcorn. There's one in particular that is hechshered by the cRc but I believe it's dairy. I've used butter flavored cooking spray before with success, it's paerve and just has a hint of saltiness.