>where would one get the seeds?
Pretty much anywhere. Garden-variety poppies produce opiates. It's illegal to grow them for this purpose, though.
A friend used to make a potent tea from dried poppies, which he got from a retail shop in Manhattan's flower district. He was quite the opiate connoisseur, too.
Not, of course, that I recommend doing any of this. It would be illegal.
Doug