Is this what you're looking for? https://superbrightleds.com
They have replacement led bulbs already configured for 12 or 24 volt, even have some for 120volt.. 8)
You simply add a series resistor to the LED, based on the current you want it to take - but from 12V you will be wasting a lot more energy in the resistor than the LED will receive - you would normally put more than one LED in series and make the resistor smaller, in order to reduce the losses.
A lot really depends on what it's for, which you don't mention, but generally an LED is a lot less bright than a lamp. If this is just a small dial lamp, an LED 'may' be able to approach a similar brightness.