Just found this one that claims to be 15 watts. Its only $27. I'm tempted to buy it to test it out.

https://www.amazon.com/Alotm-Wireless-Charging-Convertible-Charger/dp/B076CDJZ2X/ref=sr_1_16?s=electronics&ie=UTF8&qid=1513257692&sr=1-16&keywords=15+watt+qi+charger