Factory sensors are expensive, but you can use aftermarket for much less.
If the light is flashing, you use the tpms tool on each wheel, and the ones that don't communicate with the tool get replaced. It's usually just one that has a bad battery or was crushed by someone replacing a tire. Too much tire pressure can also make them unhappy. I've seen over 100psi in a car tire before, so make sure you haven't been adding air to the wrong tire for a month.