Not sure where you're located - but I use a cheap food thermometer (people typically stick it in steak to check internal temp. A better keyword may be milk froth thermometer, an example i search up on amazon.com.au: milk froth thermometer
I think that'd work perfectly if it displays farenheit/celsius, just clips onto your kettle (mine's a hario bueno so it works with the lid open)
I usually boil the water, open the lid, stick it in and wait. At ambient temp of about 5-10deg celsius, it takes about 2-3mins to get to 90deg from 97~98deg boiling water.
You can boil the water with the thermometer in as well, so you don't have to wait for the water to cool down.