For froslass non hail.
You can get it via dexnav in Shoal Cave or in XY you can try to find it in a friend safari. It's found easily in a friend safari with snorunt in it. But it's really hard to find snorunt with HA via dexnav. Good luck
You can Dexnav chain in ORAS (Shoal Cave Bottom Floor)
It can also be found in Ice Type friend safaris
An Ice-Type Safari with Snorut with all 3 Pokemon unlocked will have a chance of having a HA Snorunt
People on the DB Chat.
Sites like reddit and Gamefaq Trading Boards