LOL that's funny! I don't know, I think you have to buy them from online or at a Harry Potter convention....
Try this link the robes cost about 24 dollars and less depending on the number that you order.
L.L. Bean makes cozy ones for winter. But I'm sure you could get nice stuff at a department store sleepwear section, like Macy's or JC Penney. I don't know of any sleepwear/ lingerie stores that target older women only.