When you separate from military or federal employee service without a retirement or a family member leaves a current job, you lose your healthcare associated that employment. 

For active duty military and reservists covered under Tricare if you separate without a retirement you qualify for the Continued Health Care Benefit Program (CHCBP). The deductibles and cost shares are relatively low, but premium for individual coverage about $6,000 a year. For a family it’s a little over $16,000 a year. https://www.humanamilitary.com/beneficiary/benefit-guidance/special-programs/chcbp/

If you are a separating federal civilian employee, you can qualify for Temporary Continuation Coverage (TCC) which is a continuation of your existing FEHB policy. But you must pay the full premium for the plan you select, both your and the government's shares of the premium. So you can expect TCC to be about 4x as expensive . https://www.opm.gov/healthcare-insurance/healthcare/temporary-continuation-of-coverage/#url=separate 

COBRA coverage is available for regular civilian employees of companies with 20 or more employees. COBRA is a federal law that gives you the right to keep their employer’s group health plan after a job loss. You will have to pay the full health insurance premium, including the employer portion. Check with HR for details.
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or all three of the current employer continuation plans temporary the service member or employee can get 18 months of coverage, eligible family members 36 months.

There are other options on the Health Care Exchange at https://www.healthcare.gov/  You'll see plans, costs, and possible premium tax credits that can greatly reduce your overall costs,  Click the search box at the top of the website and enter Preview Plans. You’ll need to enter some basic info like your state and anticipated income for the year you want coverage for details. Your costs will depend on your income relative to the federal poverty level. If you qualify for the premium tax credit subsidies. You can have this credit applied to reduce your monthly healthcare.gov insurance premiums. Or you can wait to you file your tax return at the end of the year and apply it to the federal income tax you owe for the year. 

If your income is below 138% of the federal poverty level, you may be eligible for Medicaid and /or Children’s Health Insurance Program (CHIP).  But these are state based, whichcan be challenging if don’t know where you will be settling. If you qualify for Medicaid, but choose to buy health insurance on the exchange anyway, you are NOT eligible for for a premium tax credit.

For 2022,  if you earn more than 400% of the poverty level the premium tax credit begins to phase out. In 2023 it will revert to being a cliff. One dollar past 400% and you get no subsidy.

You  need to enroll in a particular Silver plan or better to receive the premium tax credit. Your costs  are based on your expected household income for the year you want coverage.  If your income doesn't match your estimate for the year it will be reconciled when you file your federal income taxes. For your household size count yourself, your spouse if you're married, plus everyone you'll claim as a tax dependent, including those who don’t need coverage.

You can also shop around with different health insurance companies or brokers. If you will earn too much for an exchange premium credit, you may find a cheaper alternative directly from an insurance company.