You could get a 12v-24v transformer. I've also read that you can use a 5v and 12v line to make 17v. I'm not sure if I remembered that correctly, but a little research will do you good.
Edit: Here's the
solution, and a good read. You can get a variety of voltages. 7, 17, 24 and a few others are possible, but you could just do 24, and use a rheo to lower it.