DIE 1 was in the December Previews catalogue from Diamond. If you want to pre-order a copy, speak to your retailer.
What’s preordering? Well, comic shops base their orders on the amount of preorders and general interest in a book, so if you want to ensure you get a copy (and help a book) it’s always a good idea to preorder. If you require explanation of how to preorder , this comic Kieron did back in 2014 should be helpful.
In practice, if you speak to your retailer and tell them you’re interested, they’ll likely order copies. Comics are a hell of a time.
If you need them, the pre-order codes for issue 1 are OCT180012 (For Stephanie’s Cover) and OCT180013 (For Jamie McKelvie’s Cover). As said, you really should be okay just be talking to your retailer. You can use this excellent website to locate your local comics retailer.