You can buy it from comic shops (find your local shop here) or digitally.

DIE: FANTASY HEARTBREAKER, the first collection of DIE is available from comic-shops now. If you’re looking for a local comic shop, this site can help. It’s also available from book stores, and online retailers.

DIE’s second arc, SPLIT THE PARTY, begins in August. If you want to pick up the new issues as they come out, you can talk to your retailer to pre-order them.

You can also buy individual issues digitally, or subscribe. You  can do so here, which is a nifty thing to do