I always purchase the thermometer/hygrometer set in the reptile section at Petco. The hygrometer is freakishly on target. So when I replace, I buy that set.
But always calibrate the hygrometer before you set eggs. Just to be safe.
This medical thermometer from the pharmacy at walmart works very well for me. I use it in conjunction with my GQF dial thermometer. There is only .2 degrees difference between them. At $9.00 it's a good deal.
Just drill a 1/4" hole in the side at egg level and insert until it fits tight...