i have a 1000 volt dc power source with milliamp current. How do i downsize my voltage to 12/18 volts without downsizing current?
I hate to sound like a smart-mouth, but the way to do this is get 12-18 volt supply.

Since you don't want to change current, a voltage divider won't help.

Since I assume it's a DC supply, a transformer won't help either.

Most importantly, you're running something at 12-18 volts, but you are suggesting also having a live source of 1 kV right next to it. The potential for disaster (pun intended) is just enormous.

Get a cheap little supply, and don't take chances.