Did you talk to the person in charge of the fish section? They may be able to order some, even if they weren't carrying them. (They have them listed on their website, so it seems like they would be available to order.) And since they're platies it seems they would still sell well, so it wouldn't be a crazy thing to ask them . . .
You might be able to call and ask to talk to someone, if it would be annoying to go back to the store.
Joy to the fishes!