It would be a lot of fun trying to find a bulb that matched the size and spec you would be looking for. I'm assuming you're referring to the construction type halide lights that have the small (half-inch thick, 6" long or longer) double ended bulbs? These are designed to produce as much light, with as little power as possible -- not to provide a specific light band for your fish tank.
However... if you can figure out the style bulb that is, you can find someone who supplies various spectrums/gasses in that bulb.
Maybe this: http://www.elights.com/usmethal15hq.html
or this would work: http://www.elights.com/us25hqidoube.html
Dont take my word for fact - I don't know, but I'm thinking that would be what you were looking for. :|