TN visa holders should qualify to buy a plan on the Obamacare exchange. Whether or not you get a premium subsidy will depend on your income. As alphazip writes, many "affordable" plans will be high deductible with copays. Obamacare is designed to make healthy young people pay into the system, often for bad coverage, so sick old people don't get denied for pre-existing conditions.
I don't know how long you are staying in the US, but if its not too long and you aren't abandoning Canadian residence during this period, see if a Canadian travel insurance policy might make more sense. Probably won't if you will be working in the US, but worth comparing, if your situation warrants it. Good luck!