Kink talk and getdare aren't only sites, you have lots of sites about bdsm so you can always try there. And you look inpatient which isn't a good virtue for a Dom/Master. You can always find someone for quick fun, but girls who doing that just come and go. For serious and long term relationship communication, bond and trust between two sides are important and for that you need time. Also, problem for us straight Doms/Master is lack of female subs/slaves. And lots of them are fakes, trying to blackmail or get some money for they service (camgirls). I'll be honest, i didn't find here or on getdare any of my previous subs, from time to time some girl come just to explore is she into this or not, and all cause of that book 50 shades.
Some hints from me.
You can put very nice ad, lots of people will comment but since you are 18yo, they will not take you serious. Good ad attract women who are older than you and know what they want, but again they see how old are you and very few like to be dominated by younger male. Very rare some young girl your age is interested in that. Younger girls are more how to say open minded (don't want to use term sluts) and they want something quick. I know lots of people here will not agree with me, but i am long into this, real and online and i know what im talking from my personal experience. My posts depends on what i am looking for. They can be very short when i just want short term fun or very detailed if i want long term relationship.
Also your technique as a Dom/Master is important. Every man have his own way, and all what can i say to you, with age comes wisdom, so you still need to develop yourself as a dominant person.
Be patient, be unique, be yourself and you will find someone, or she will find you.
Master Vagrant