There's no easy answer, but assuming you can't get it through a spouse/partner's group medical plan or through something like a professional organization, look at the healthcare.gov website. It has a portal to information about individual plans available in each state - as well as a host of other info about medical insurance.
How much the premiums will cost can vary a LOT by location and the type of coverage you choose. Something like a high deductible health plan might offer the lowest premiums, but you will have to plan to pay out-of-pocket until the deductible is met (a Health Savings Account can at least help you save on taxes in that case). Depending on your location, sometimes there's an HMO plan available that offers good value for the premium.
One positive: if you have freelance income (filing a Schedule C, for example), you have a good chance of being able to deduct the premiums on your taxes.