Q:
Ghostly Outfit
A:
Only ArenaNet. You can follow the Sticky above, log into the Gem Store daily, watch the News Blog, and/or use one of the apps that will alert you to offerings from the Gem Store.
Good luck.
It could very well come in any random sale, but I’d hazard a guess that the most likely time will be during the Halloween event in October. Keep an eye out between now and then but most particularly watch for it then. It is a seasonal sort of outfit, after all.
It was on sale about 1 Month ago. So you prolly have to wait quite a bit.
gw2timer.com has a notification tool that watches the GW2 API for items on sale in the gem store, and can alert you when something comes back.