As someone who has approached the problem from both angles, let me tell you that Brian is right. There are a lot more available, relationship-quality women out there who don't know that they'd be open to diaper play with the right guy, versus available, relationship-quality women who are into diapers. Taking the risk is hard, I know. I've done it a few times. And then came the time that it paid off. My wife adores me for me, and we've both been wearing diapers all week. She never heard of it before getting to know me.
Oh, and in regards to the "when", I will simply tell you what worked for me. My relationship was long distance, but we skyped daily. I told her via a well-thought-out email after it started to become serious, after I had had enough time to establish that the compatibility was very high, and the time was "right". That is, too much longer and it would feel like I was keeping secrets. I had been showing her all the pages of my book, and it was time to show her that page. It was a hard email to write and send, let me tell ya. I was so happy with the ultimate results.
Just talk a lot, and don't be in a hurry to jump into the sack. Patience really pays off. Lucky me, I had to. She was thousands of miles away for starters. :)