I would just say "Hey, I think you're cute, can I give you my phone number?" and offer my card. He'll either say "No, sorry, I have a girlfriend" or "Sure". Either way, he'll be flattered, so you shouldn't have to switch grocery stores. And actually, even if he rejects you flat out and you still hold you head high and act friendly while shopping there, you're flashing major confidence signals.
I'd bring up the poly stuff on the first date. TMI before then.