I don’t think it’s possible to buy it directly from DC.

I think DC prefers people rent their comics (via DC Universe Unlimited), much like cool kids today seem to think you're square, maaaaan if you actually buy your music, and double square if you buy it on physical media.

(I think I crashed the brain of some poor shopcritter at an Apple Store when I told them that not only did the iTunes Store not have all the music ever made by humanity, but there were alternate ways of getting music onto one's device. I hope they recovered and went on to go outside occasionally.)