You don't NEED a thermometer; basically it has to be hot enough to steam, not hot enough to boil. I've never used a thermometer when straightening my wigs and no damage was ever done. This assuming your wigs aren't heat resistant (if they are, then you're better off using heat tools like flatirons and blowdryers). If it's mostly frizz, gently combing and conditioning the wig and snipping off the strands with the worst damage should be done before you try hot water; hot water is mostly for straightening or fixing curls and kinks, it won't do much for frizz. And wash all the product out of them too, so you have the unstyled hair itself to work with and can style it from the beginning again.
Originally Posted by Moofingham
There is a vibe here that says "We're in this together! Through thick and thin fabric! Through cold water washes and burning hot irons! Though we might super glue ourselves to our projects, cut holes/gashes/oh-god-mom's-gonna-kill-me into the dining room table, we will stand strong together. Unless there is a 75% off sale at the Fabric store. Then you're on your own. And get the hell out of my way." <3