It hasn't been stated how, but he could have made it himself.
Also, it was stated that Shin was a fan of the Uchiha Clan and especially Itachi and that's why shin has the Uchiha surname and got the Akatsuki robe in the first place. Also, Shin's robe doesn't look like the classic Akatsuki robe we all know and love. It only has two Akatsuki clouds on it and it is a lot shorter than the original Akatsuki robe. The only similarity is the cloud emblems and color. It really just looks like a fan who tried to make an Akatsuki robe and ran out of cotton and material to make the robe.
