Someone else mentioned the Astron converter.
You can also buy inverters (usually online) designed for 24V input, and use that to run a small gel cell charger, with an appropriately sized gel cell. Neither the charger or the battery need to be big, unless you need to transmit a lot. The charge "refills" the battery when you're listening, or when the radio is off. Resistors are a bad idea, and tapping power from one battery is another bad idea, not just because of the damage to the high side battery, but the transients in a 24V system are double that in a 12V system, and your 12V radio has likely not been designed to live in that environment.
