Depending if you want to have something specific based on your ideas, or if you would take some ready stuff also - I made very good experiences with this guy:
https://www.facebook.com/headsplitdesignofficial
Bought the last album cover as well as some shirt designs from him. you can find his designs also very often with Bands like Origin, Terrorizer, Beneath The Massacre, Psycroptic, Pig Destroyer etc.
He has a very specific kind of style, so you either like (almost) everything he does or nothing at all.