I save the little packs from vitamins, electronics, and shoes. Only issue is that there’s no easy way to know when to reheat them.
I also use the color coded ones that can be recharged. Mine are blue when fresh/recharged and turn light orange when depleted. Sometimes it takes 3 months for them to turn orange but that depends on the moisture in the can (cardboard boxes hold moisture) and the can’s seal. Once most of the packs are orange, I just put them on a cookie sheet in the oven on 215 degrees for an hour, then they are magically blue and fresh again.