Whilst it is now legal in various parts of the US to grow a completely different kind of weed, we won't offer that as advice, because in most places you could still get in a lot of trouble that way! Catnip, however, is a weed that grows easily, and is in constant demand everywhere. You could become a catnip farmer with a start-up cost of less than $5 for the seeds, so long as you have somewhere to grow the catnip.
If you're not a cat owner and you're not familiar with the substance, cats go absolutely crazy for catnip. It's used as a treat for them, and sometimes it's placed inside their toys to keep them stimulated. Almost everyone loves cats, and they're never going to stop being household pets, so there will always be a demand for catnip. Who's going to do a better job of supplying it than you?