I use a bag of rice. Heat it in the microwave until it is pretty warm to the touch but be sure to put a towel over it before you put it where the kittens are. I think they make packs specifically for this purpose but it is probably expensive. If you can get one from a pet store that would be ideal but the rice pack should be fine.
It's not a good idea to use a heat pad because it might get to hot. If you use one though, just be sure to check it often and preferably use one where you can control the temperature, like, low-medium-high heat and put it on low. It may not feel to warm when you touch it but the kittens will be laying against it for a good length of time and it will get warm to them, their skin is more sensitive. Make sure they can move off of it if they need to.