Alliance players must first complete the quest chain beginning with An Explosive Opportunity before this pet will appear on the vendor.
Unlocking this vendor will require you to participate in Pet Battle PvP on your Horde character for a chance drop of the item A Shady Message. Using this item will unlock the mission A Shady Character at your Mission Command Table. Once you complete the mission, Olly will offer various Alliance-Only pets.
As of writing this, it is unclear what pre-requisites you will need post-Battle for Azeroth for A Shady Message to drop. If you are looking to unlock this vendor, we strongly recommend having your Mission Command Table unlocked on your Horde character, which is part of the introductory quest chain for Battle for Azeroth. It may also be a requirement for you to queue and remain in a Battle for Azeroth zone for this to drop, but that is not confirmed, it is simply a theory and suggestion on our part.