You’re going to be using a resistor to change the current; your best bet is to: Contact an electrician and/or electrical supply house and tell them your needs. They will want specifics on the household current and the required current the lamps use.
I done a similar job, 120v-24v and these lamps last forever.

If your under cab lights have a transformer (ie:small box on cord) mount an outlet in one of your cabinets and plug it in or wire it to the box.