These are the exact same ones that people use in coastal, humid parts of the country. You don’t need to spend a lot of extra money on something specifically designed for a greenhouse unless it comes with extra, useful features such as CO2 monitoring. If the humidity is above 75–85%, your plants will have difficulty breathing. Since you’re basically filling a room with tubs of water then pumping oxygen into that water, a dehumidifier can go a long way towards keeping the air merely moist instead of downright swampy.