Is it illegal to buy used Cats in other States too
I ask because it is here in Cal.

Yep, it's a FEDERAL law.

That said I know from experience with other vehicles that a cat doesn't care what vehicle it's under and most modern cats flow about the same as high flow cats do.  I know someone who's running cats off a wrecked big block Suburban on their Ranger.  Most junkyards don't care what you take out if you pay for it and most emissions places couldn't tell if the cat on your truck was used or not.