how can i find the snow witch sets?

Go through Ranni's quest and you'll get them eventually (you can find it if you go to the northwest corner of Liurnia, the Caria Manor area)

Once you give Ranni the Fingerslayer blade, she will leave her location, and the seal will open to Renna's Rise. This is located just north of Ranni's Rise. Once at the location, climb up the ladder. Before going outside on the second floor, turn around and there will be a treasure chest that holds the Snow Witch Set