You can buy this, there are different levels of pricing based on what you want or can afford. 50% of all profits go to The Andrew Breitbart Children's Trust.
It turns out this painting was plagiarized from another one done for the video game Assassins Creed. h/t doktorzoom in the comments for the heads up. I looked at the post he links to (by the good "doktor") and I think it clearly plagiarism. Regulars to this site know how I feel about that being a victim (so to speak) of it myself fairly regularly.
To play devil's advocate (but this should in no way be construed as justification) it's not a apples to apples example of plagiarism. The Breitbart painting had quite a bit of the details in the armor changed to make it look original, and while it's generally frowned upon this type of copying is not unheard of in the comic book world.
That said, the post will stay up, because regardless I still like the image. However, the store putting it on sale has removed it, so that link is now gone.