For most Medicare Supplement plans, you can visit any doctor or hospital that accepts Original Medicare. Your Medicare Supplement coverage will kick in after Original Medicare has paid its portion of your care. You should not need referrals to see a doctor.
The only time your choice of Medicare-accepting doctors and hospitals is restricted is when you choose to purchase a Select Medicare Supplement plan. Select plans have lower premiums than their traditional counterparts, but they have a network of care determined by your insurance company, so the locations and doctors covered by the plan are limited except in some emergency situations.