I don't understand. Medicare Supplements are portable, at least all of the United Health Care supplements sold through AARP. F, G, N plans cover your pretty much anywhere you go, with different deductibles and co-pays. I'm still on a Plan N because at this stage of my life I prefer the lower premiums and am willing to pick up the co-pays. It covers me anywhere in the world, where Medicare does not, but I still buy travel medical coverage when I leave the U.S., for med-evac and to take care of insurance requirements for countries that have national health systems that don't take care of visitors (e.g. Czech Republic).
As you approach Medicare age, most folks approaching you to sell you something are selling Medicare Advantage programs, often for no premium. These are often essentially HMOs, and they are often geographically restricted and provider restricted.
Going into my 72nd year, I'm paying $110 a month for Plan N in Oklahoma.