You should be able to get them on Ebay. Just need the bulb type. The ones I have are OEM and silver frosted so you can't tell they are amber until they flash. They are like Solaris bulbs with silver paint on the top part of the bulb. I believe you can order the Solaris bulb in hyperwhite, or amber although both are silver when not illuminated. I would check with Bimmian.
http://www.bimmian.com/Z4/SOL/