Monorail or boat to MK, then take the CSR bus. Boat might be easiest, as you can stay on the beach until you see one coming, then walk down the dock and hop on. Do check park hours to see how late MK transportation runs. It's usually an hour after the park closes.
Very worst case scenario is to take the bus from Poly to DTD, then hop the CSR bus. That usually takes a looooong time. If you have to do that option, better to cab it if you didn't intend on shopping around at DTD.