what stores sell metal roofing?

try lowes or home depot
It depends on what kind of metal roofing you will be using it for, barn, house, shed etc. Most farm stores sell fencing and metal roofing. If it is for a house, try Lowe's.
If you are looking for standing seam roof's you probably aren't going to find any at a home store like that. they have to be professionally installed. if you mean like corrugated sheet steel like you would maybe use on a shed or patio maybe a farm type store like the other reader suggests.