What are your current requirements? A 78L24 can produce 24V @ 100mA. An LM317K can produce between 1.2V and 37V @ 1.2A. An LM317T can produce between 1.2V - 37V @ 1.5A. An LM338K can produce between 1.2 - 33V @ 5A.
You can use any of the above with an external pass transistor. Check their datasheets. That would be a whopper of a transistor, though, because you're looking at 84 Watts of power dissipation just across the transistor. Think "switcher."