I was in the same predicament about 6 months ago. I have two 600w Lumatek Digital Ballasts and I wanted to be able to use them for vegging the first two weeks. Of course, I could have just used the Hortilux 600w HPS bulbs that I had, but I wanted the shortest plants possible, so I preferably wanted to use MH for more blue light. Anyway long story short, I found a company who is the only one I know of that makes 600w MH bulbs specifically made for 600w Digital Ballasts. The company is called Sunpulse. They can be found at
www.sunpulselamps.com
Here is the direct link to the 600w pulse start MH bulbs is here --->
http://www.sunpulselamps.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=1&category_id=1&option=com_virtuemart&Itemid=6
I'm not sure where you're located, but I was lucky enough to find them at my local hydroponics shop (Canada). I'm sure you could email them to see where you can find them in your area (or order them online).
They are great bulbs, BTW. You can get them in various spectrums from 3k to 20k. I have the 6.4k version which is supposedly best for veg. I've been using them for over 6 months now and they work great, and they aren't a compromise like the conversion bulbs (you get more lumens) and they are cheaper (I paid around $100 CAD each for them vs. $160 for the conversion bulb).
Let me know if you have any luck getting them