Supplemental insurance was set up by the US government to fill in the gaps that Medicare does not cover. The plans may be offered by private businesses, but the coverage of each plan is laid down by the government. You have several different levels of coverage to choose from, plans A through L, but these plans are uniform across the nation. The best supplemental insurance will depend on your needs, but your location will not have an affect on the coverage that you have to choose from.
Since the coverage is the same no matter what your location and no matter which carrier you choose, the only thing that you will have to consider will be which plan is right for you, and which pricing scheme makes the most sense. The most inclusive plan is Plan L, but the premiums will undoubtedly be higher. You have six months to decide on a plan that makes sense for your purposes. During that time, the carriers can not ask for medical screening and they can not deny you coverage.
Individual premiums will vary, but they pricing scheme works in one of three ways. It can increase with your age, it can be based on your age when you sign up, and it can be based on your location. If it is based on your age as you grow older, it will be most expensive in you later years, and cheapest when you first sign up. If it is based on your age when you sign up, it will not increase as you grow older. The same is true when the pricing is based on your location. The choice between these last two options will depend on your age when you sign up.