If you can find insurance, it will be pricey. Unless you are eligible for Medicare, and you might still consider that pricey. Adjust your expectations.
When I retired at age 58 to travel, I signed up with an Aetna "Traditional" plan that covered wife and me nationally (80% with very high deductibles and out of pocket maximums) for about $2200 a month premium. When traveling outside the U.S. we also bought travel insurance with emergency medical coverage. Premiums are high for "take it anywhere" coverage using nationwide networks, and it will never be 100%, can be as low as 50% for out of network.
So leading up to Medicare, I paid almost $200,000 in health insurance premiums for coverage we didn't use for five years, until my wife's terminal illness, which cost the insurance plan at least $900,000 and cost us another $150,000 over three years for deductibles, co-pays and treatments not covered by insurance.
Since her death, I've been on Medicare, paying about $6000 a year in Part B, Part D and Supplement premiums, deductibles and co-pays. So far, Medicare and the supplement pay almost nothing because I don't need much medical care, it all falls within deductibles.
I know things have changed with the ACA, for those not on Medicare, and the nationwide coverage options are probably fewer now, but when you find insurance for travel, be aware that it will not be cheap. To meet the "affordable" part, insurers have moved more into HMOs, localized providers, and managed care. Carte blanche will be expensive, and if you find an insurer that still offers that type of policy, premiums will be very high and coverage less than 100%.
If not yet on Medicare, you need to find an insurance broker and explain your needs, but expect to pay high premiums.
If on Medicare, taking Part B and an F supplement (UHC covers nationally) will provide the most complete coverage, N supplement will cover about 80% with slightly lower premiums.