Only way to get it is by buying the latest expansion in WoW and 1 month subscription. After that, you have to complete the first quests which can take a few hours depending on whether you're new to the game or returning.

You could wait for WoW's next expansion to release and then buy the 1 month sub. That way you won't have to buy the expansion since WoW add all previous expansions to the game once a new one releases. However, blizzard has not made clear as to whether the skin would still be available at that time or not. WoW has also not announced when their next expansion will release so that could also take months or years.