the 5000K D1S bulb is a good choice. it's not a popular bulb with the aftermarket, that's why it's harder to find. a quick search on google will lead you to some suppliers.
i would contact a supporting vendor on this forum:
http://www.300cforums.com/forums/xenon-depot/
my personal company imports HID kits direct from the manufacturers, so i'll give you some advice on what to look for when buying:
1. warranty, at least 1 year.
2. return policy (do you have to ship the bulb, or can you take a picture of it and they will replace).
3. shipping from where? overseas, or US seller?
all the best regarding your upgrade.