Cannot really think why you would want a neon on 12v, rather than a led. A simple oscillator and step up transformer will do, you should only need around a milliamp at 90v or more. No need to rectify. Not sure what transformer you would use though, perhaps something from an old transistor radio would do. Not aware that there are any circuits around for this.
Hi:here you have just one of many videos about how to run neon's with 12 volt at low amps and the same time to charge another batteries with residual energy