All such good advice! The real question, to my mind, is whether you are seeking a poly arrangement or something more akin to a kink arrangement. If you have a full on sexual as well as emotional relationship with your Mistress then a poly arrangement is what you are seeking. To that end it would do you well to look online for poly groups in your area and/or some sites that cater to poly people such as ok cupid or 'Loving More'.
Still and all the best way to find your special someone (or someones!) is to get out there and look just as if you were monogamous! The only real difference is you MUST be honest from the very begnning of any potential relationship about your desire to be poly. It's the ethical and sane thing to do. Now I don;t suggest you wear a sign or blurt out your desires but defnately before you form a deeper commitment (or have sex!) be sure you are clear about your preferred arrangment.
If your relationship with your Mistress is purely kink then you also need to be clear to any potential lifemate that this is something you will continue to pursue and give the new love the choice to stay or go, again it's the only ethical thing you can do.
Good luck with your search and with your long distance relationship! ~Airen