Can you purchase a butterfly?

I'm curious, can one actually purchase a butterfly? Is it even legal to do so? I've heard of people keeping them as pets, but is it feasible and ethical to buy one from a store or online? Additionally, what would be the considerations one needs to make before making such a purchase? Are there any specific regulations or laws that govern the trade of butterflies? And finally, how does one ensure that the butterfly they're purchasing is healthy and well-cared for?