Forced-air heater for old floor registers
Toe kick heaters such as the slant fin sidekick are fan assist hydronic heaters as far as I know. Any heaters that fit in the register space that I've seen were electric, and that is expense to run and won't deliver much heat. They do exist so depending on your climate they may work for you. Dimplex makes one but I think it requires some installation. I've seen some that simply plug in. I just don't know who makes them.