Well, answering that can be a challenge, after all it depends where you are located, the fact that you want an "open" relationship (I dont think many mothers would like to have that kind of relationship, but everything is possible). You can look on your community, going places (after all love comes in the most random occasions, right...you can meet the woman of your life while shopping for groceries...) You can also look on Tinder and other websites, though I am not sure you would find "a mother"...my advice is dont restruct your searches to who is or is not a mother...also keep in mind that love cannot be picked like we pick food in the shelves of a supermarket...