Is there a specific enemy that drops it?
No try use grinder
…and always check vending machines. I’ve seen two Avalanches as items of the day. That said, don’t farm vending machines for that specifically; just note that they’re worth a glance when you step into a map with fresh ones, even if you don’t need ammo.
Odjurymir drops the Avalanche but he only comes out at Christmas whilst the Mercenary Day Hot Fix is live. Outside of Mercenary Day, all you can do is farm the Grinder, Vending Machines, the Sentinels and EOS.