If you are younger than 65 but you have Medicare due to disability, insurance companies in about half of all states are required to offer you at least one Medicare supplement policy. However, those who have Medicare because they have End-Stage Renal Disease (ESRD) are often not able to purchase a Medicare Supplement until they turn 65.
Rules can vary quite a bit from state to state. Companies may be able to use medical underwriting if you apply outside of your open enrollment period, so enrolling in a Medicare Supplement policy when you are not a senior may lead to higher premiums.