Comic companies have been releasing books with blank covers. Usually they are somewhat keystone issues, like a first issue or a landmark issue. They are easy enough to find nowadays. Marvel, DC, IDW, Image, Dynamite... everyone seems to be putting them out. If you can't find a comic store near you with one, hit eBay.
My first blank cover was a Hit Girl #1. I kinda cringe when I see it lol But it was a stepping stone.