Not everybody is permitted to buy a medicare supplement plan at any time. Whether or not a specific plan is available to you, and what the terms are, is contingent upon when you apply for the plan and what other coverage you have had. An understanding of what these restrictions are will make it possible for you to take advantage of the medicare supplement plan that works for you.
If you have signed up for Medicare Part B in the past six months, than you are in guaranteed enrollment period. This means that you can sign up for any medigap policy you want to. It would be possible for you to sign up for Medicare supplement plan D without having to go through any medical screening. Any company selling a medigap policy that you are interested in that resides within your state would be available to you. The company would be required to accept you, and would need to provide you the same benefits as anybody else who signs up for Medicare supplement plan D. They would also need to offer it at the same price as everybody else.
If you had a Medicare Advantage plan, and the company decided to stop offering this service or went out of business, you would not be guaranteed access to Medicare supplement plan D. You would be guaranteed the right to sign up for Plans A, B, C, and F, however.
If you did not take advantage of a Medicare supplement plan during the first six months that you sign up for Medicare Part B, you are not guaranteed the right to sign up for a plan. You have the option, but the insurance companies now have all of the deciding power. They might require you to go through medical screening, for example.