If he knows the fabric, but not what it is called, then there's probably a bolt or two of it in that storeroom. Even if there's not, his dad there probably know not only what it's called but where to get it.
But how long will it take for him to realize that.
He obviously already asked his dad offscreen /s
And the name of what he wants is obviously vantablack, which if a color, and not a fabric type. They should look at
extremely specialzed paint-stores.
No, that's a texture (ish. technically it's a specific cloth, but the texture of it would be the reason a cosplayer would pick it), and all the panels in the manga has only conveyed color so far, nothing about textures.
Besides, you would think that he would either know it's name, his granpa would, or any of the numerous clerks they have consulted. It's commonplace enough even random comments know the term, while not entirely unelated to the clerks they are asking.