12v led to be run at 24v??

Anyone know the value of resistor I'd have to run in series with a 12v led so I could run it at 24v? Or a link to a 5mm 24v led?

http://led.linear1.org/1led.wiz

http://www.technobotsonline.com/opto-el ... ators.html

24V ones are in the bottom half of that page. They're 5mm but with a casing surrounding them that you can use for mounting them. I think the casing is roughly 8mm diameter.