The GTS would involve trading, but it's a very easy way to get version exclusive Pokemon. Since Sun players can't get Sandslash, if you offer one up on the GTS requesting a Ninetales, the trade will happen pretty much right away, since it's a win for them too. The process happens by itself, you just submit a Pokemon and wait basically. You'll probably find good offers doing the reverse and looking for people requesting a Sandslash as well.
If you want one with good IVs, ask for a Vulpix and set the level to 1-10 and you should get breedjects with 4 or 5 perfect IVs, and other cool things. The GTS can get you really nice stuff, more people should try it out.