Red Baron Corki

Its a retired skin, so it is no longer available. There are some skins that are available via Mystery Gifting despite not being available, but I'm not sure if this is one of them. And obviously, you are not guaranteed to get it since its random.

I received Annie in Wonderland via Mystery Gift, and I believe it was retired around the same time, so there's a good chance that is the best way to get Red Baron Corki. There's also a chance that Riot may re-release it sometime in the future, but I wouldn't count on it.