If you only have Medicare, the Medicare insurance premium is listed in box 3 of the Form SSA-1099. TurboTax automatically enters this amount for you.
If you have a Medicare Advantage Plan or similar plan where the premium is deducted from your Teachers pension, that amount should be listed on your year end summary.
If you have a Medicare Advantage Plan or similar plan that you pay for your self, those payments would appear on your bank or credit card statements, depending on the form of payment that you used. In this case, you should also be able to get a statement from the insurance company.