Well, you could take the 36V line, and connect it to both the device needing 18V and to a resistor hooked to ground. The resistor will need to be equal in resistance to the object needing the 18V. You can find this by dividing 18 volts by how much current it draws (in amps) when connected (should be able to find this somewhere). The resistor will have to have a power rating of at least (18 * current drawn in amps). I'd go for more though.
Here's a quick and dirty schematic:
|----------- <device> ------- <ground>
|
+18V o---|
|
|----------- <resistor> ----- <ground>
JigPu