Players on the PTR have already discovered where to find the Nightsaber Cub and the Winterspring Cub! This leaves only the Panther Cub without a known home, but there's still plenty of time to poke around on the test realms.
Nightsaber Cub - New TCG Loot
Currently on the PTR, Landro Longshot has the Nightsaber Cub listed as a TCG redeemable item. The trading card expansion deck in which the Nightsaber Cub will likely be featured as the common loot card is called "Twilight of the Dragons." According to WoW TCG Loot, this set is due for release in late June.
Winterspring Cub - Updated Alliance Mount Quest
As the name indicates, this item can be found in Winterspring on the PTR. At the moment there are two Winterspring Cubs available, however, only one is a true vanity pet.
One cub will be part of the revamped Winterspring Frostsaber mount quest chain. Much like the Horde Venomhide Ravasaur mount dailies, the Alliance will have a baby Winterspring Cub to care for during their quest for a Frostsaber mount. This cub cannot be used outside of the questing zone, and it is not a trainable spell.
Do not despair, though! There is still a possibility that an actual vanity pet version will be available. A vendor located in Everlook on the PTR sells a Winterspring Cub for 50 gold. Hopefully she will still have enough cute little cubs to go around once Patch 4.1 goes live.