What's the best way to purchase a Part D Medicare supplement, and what is the cutoff period for switching providers?

I have had two Part D Rx medicare supplement insurance policies for my mom in the last couple of months. The first was bad, the second even worse. In both cases the reps with whom I'd spoken lied about the cost of meds when we went over what she was taking and what the price of the meds would be. United Healthcare told me one of her meds would cost $25.00, when I went to the pharmacy they said it would cost $295.00. Is there a cutoff period for switching providers?