What kind of headlight bulb do I need for a 2006?

Prius is brand new to me, I have no idea how to describe it except that its the kind with the ballast that takes either a D4R or D42...I guess?
The headlights work normally for a while, then the driver's side just blinks out. Then its back on the next time I drive...I don't know. I'm sure hoping its just the bulb and not some issue with the ballast. So I figured I'd try replacing the bulb first...and then have gotten so much mixed information on the type of bulb after attempting to Google for it that I have no idea what the answer is now.

Also, I was told that replacing the ballast and bulb would be some $1600...is there some way that I can switch these headlights to the regular ones? Some kind of conversion kit that wasn't too complicated? I just bought this car and it seems like a bad engineering decision to me, which surprises me considering its Toyota...is there some benefit to leaving the headlights as is?
Thanks, I'm so lost with this car...I've never owned anything like it and have zero idea about much of anything, really!