I got some off ebay, but most people reccoment HIDs Direct or something... you'll need D2S bulbs. I always think it's best to buy a pair to avoid colour difference.
I think factory fit Xenons are 5700k colour temperature, if you want something whiter people go for 6000k, I personally have 8000k's. (Got them off ebay for £40 for the pair, they haven't let me down yet).
However, I suggest swapping your starter packs from one side to the other to make sure the problem doesn't move with them, as it's not definately going to be your bulb. If you have the time I'd do the same thing with your ballasts too. If the problem moves you'll know which component you need to replace.