The cheapest place is online, if you need a D2S bulb straight away the only place you can really get them from is dealerships, and it can be anything between £80 to upwards of £100 for a bulb. I had a set of unbranded ones in my R32, both S3's and A4 and they didn't give me any problems at all. Got them for £30 so the difference in price was well worth the extra day or two I had to wait until it arrived.
Try aceparts.com.
whats the difference between the bulb and the ballast guys? what are the original bulbs K temp wise?