Allelectronics has a "HALOGEN LAMP ASSEMBLY W/ AUTO LIGHTER CORD" for $5.65. http://www.allelectronics.com/make-a...ER-CORD/1.html The catalog copy says:
Tough, heavy-gauge plastic assembly with a 12V, 5W, MR-11 halogen bulb. Originally part of a bicycle headlight assembly, it would also make a fine auto map light. Plug into your auto cigarette lighter, and you're ready to go. Lamp has an adjustable metal clip. Lamp housing has rubber O-rings between cord and lamp socket and appears to be water-resistant. 1.65" diameter x 2.08" long. 10' cord with cigarette lighter plug
It somehow seems like cheating on this thread to use something that was "originally part of a bicycle headlight assembly" but I suppose one can break out the soldering iron to replace the lighter plug, etc.