I’m fairly certain that Laertes doesn’t mean this literally. I think he means it as a kind of expression of karma – that he mixed the poison that killed him, he set himself up. But given Claudius’ proclivity for poisoning people, I am still very much enamored of the image of him in a lab, mixing up compounds and trying them on small animals, just like the Queen in Cymbeline.
He might not be mixing these compounds himself. Maybe, like the Queen in Cymbeline, he has a helpful assistant who brings poisons for his collection. This strikes me as fairly risky, however. If you’re going to commit regicide, you probably don’t want someone who knows you bought king killing drugs because he sold them to you.
I suppose Claudius could have disguised himself to visit the apothecary or just visited apothecaries around the world in places no one would recognize him.
My favorite method of procurement, though, is Claudius mixing his poisons himself – the literal meaning of this line.