A couple weeks ago, I was reading a book about it, trying to research before even bringing it up. There's a list of questions that I was going over and one of them stuck in my brain. I thought about it for a while, then asked my husband what he thought. That just got the ball rolling on what I was reading, why I was reading it, and how I've been feeling lately about 'us.' He was pretty surprised when I brought it up, but he didn't take it badly. By the end of the conversation we were making jokes about double dates and stuff. No decisions were made.
We've had a few more conversations about it since then, one going really badly, and the other a little better. Still no decision has been made, I'm not really expecting one too soon. My husband said he'd read the book and we'd go from there.
It should be accessible in Canada! You can find a list of countries where Feeld is not currently available here, Canada should be just fine!
I urged my boyfriend to get on it for exactly that reason, it was difficult to meet new people who are like-minded for both of us but especially him. Before Feeld we were hooking up with friends or acquaintances, and Feeld has given us a less messy avenue of exploration. It really has been a great way for us to explore both separately and together.
Suggested read (listen) for the both of you: Come As You Are by Emily Nagoski. Most of the time, sexual language is lacking in relationships, thus messages get lost, misunderstood, or never shared. Up your brain sex game!
I think you are looking for more "polyamory" than "open marriage". But this is a book often recommended for people who want to open up relationships: https://www.amazon.com/Ethical-Slut-Practical-Relationships-Adventures/dp/1587613379
My advise? Ensure she understands that you are committed to her and love her, and that you only want to love (but not commit) with other girls.