Replacement HID Bulbs?

[BoOsTiN]

Hello All,
Hoping someone can point me in the right direction.
I have a Catz HID kit on my MSP. I just had a bulb go out so im looking for a replacement.
Not having much luck googling around but im not too sure if im doing it right.
Its H4 4500 with the HI/LOW.
Any help is appreciated.
Thanks,