Amber LEDs seem to put put fewer lumens and seem to be more heat sensitive (lumens drop with heat faster) than red or red-orange (stats for three different LEDs). These interested me at one time. too.
I suggest red-orange as easier to get, brighter less heat sensitve. The only reason to go Amber is on the front and there a flashing white will outperform it many fold.
There are red-orange P-60 modules. Someone showed one from DX on here, maybe in the 'design a commuter rear light thread'.
If you still want amber maybe someone knows of an amber one. Or you could serach DealExtreme.com, Kaidoman.com (I think that's right), or Google 'P60 flashlight module amber' and see what happens.