Autozone is where I have bought my t-stat's from, now that you have stated issue's out of them, if this one fail's soon I'm going to spend the coin on a EMP brand t-stat and hopefully be done with it!
Try to stay around 180,that`s about what the factory recommended during that time period.Anything less & you are wearing out your cyl walls,200 & above it`s guides,seats & valve seals.