Keldeo has not yet been available to Sun and Moon, meaning that you will have to transfer a Keldeo from one of the previous two generations.
A Keldeo at Level 15 was only available through four distributions: in Gen V, a GameStop promotion event in America, Europe and Oceania, as well as a Japanese movie pre-order gift. In Gen VI, it was a gift for purchasing the South Korean ORAS guide and it was available through the Japanese 'Pokemon Scrap Campaign' distribution.
So obtaining your desired Keldeo is certainly not an impossible feat! There are some Level 15 Keldeo floating around in Gen V and VI. As below commenter said, GTS may not be the best place to look due to most event Pokemon holding a ribbon that restricts them from being traded over Wi-Fi. You can still get one by initiating a direct trade with somebody with their friend code, you just need to search for somebody willing to offer their Keldeo. Good luck!
Hope I helped. :)