How would I get such a boost driver for ne 21700 and XHP 50.2
What diameter?
Kaidomain sells a lot of drivers, as many other stores
About 22mm
You can now get the XHP50.2 in a 3v variant. So you may not need a boost driver.
Something like THIS ?
Aliexpress sells a ton of them. If price is an object, then get it from China. If you want a good deal, get it from Aliexpress like Yokiamy listed. I’ve bought several drivers off there with good results. Convoy Store is really good. Fast shipping. You can go Kaidomain too, or Gearbest, eBay. Mtn electronics and Taskled sells top-notch drivers. You can get one from Lexel on this forum as well. Their boost drivers are more expensive but will be quality and guaranteed to work to spec.
If you get the 3v xhp50.2 you can save money and get a linear driver or a Fet+1 7135. Those are like $6 compared to $10 and up.
Oh okay thanks for the answers
What would you choose?
I think i will use one 21700 Cell for the Light
But i am flexible because i have Metall Maschinen
Here is a powerful and efficient boost driver (check this Testing boost drivers' thread). Comes stock set at 4.8A in “turbo” so more ;-) brightness (3A version available in store).
Current H1-A is reported to be downgraded 
(smaller inductor), and loses efficiency. Bad choice, CDM, very bad.
I look for high quality lighting in XHP50s (high CRI). I don't care about 3V XHP50.2 variants thus.
Whats bad on the xhp 50.2 3v ?
Nothing under 5400K, and I haven’t seen anything with higher than 70CRI
Agree, the 3V version has a ugly yellowish/greenish corona.
Dedoming it helps a lot though 
Check it in its datasheet. All dies in parallel variants (3V, 1S4P) are only available in 5000K to 8000K CCT's, without any minimum CRI (much less EasyWhite bins).
Whether this is a problem or not is something I can't answer for you. 