A well meaning friend bought me one of those in the ear thermometers (but my kids screamed like I was trying to suck their brains out when I used it).
I used to sneak and do rectal ones while they slept (but that felt weird...taking off their pants when they were asleep...weirder as they got older)
My pediatrician uses one that you swipe across the forehead but I don't know if they sell those over the counter or not.
What we finally settled on were digital strips that stick on their foreheads and project the temperature (they sell tham at Walgreens). The boys liked wearing them when they were sick. If it read a certain temperature they got to do certain things...this helped with medication as well. They took their medicine to bring down fevers willingly if they could watch the numbers change on their foreheads (101...they got an organge juice smoothy...100...they got to sit up and watch a movie...99...they could have a sucker etc.)
Good luck.