I'll second the Fetlife suggestion. It's not my favourite site in the world, some people on there are really hardcore and pushy when it comes to ABDL, but there are some genuine AB's and DL's and even caregivers on there who are open and friendly.
I met one of my first AB friends on Fetlife and I've had a couple of playdates with him, and I met a DL and another close friend and confidant on Fetlife as well, this DL friend is local and not really into the baby stuff, so we just have a regular friendship where we are open about diapers and wear around each other.
Fetlife is also good for finding munches. Munches and ABDL get togethers are usually very discrete and private and are always on the level, you can meet some local friends through munches if there are any in your area and if you don't make any real connections with any of them, at least you'll have a good time playing games and chatting with other littles.
I no longer use Fetlife because I am mostly a nonsexual AB and there aren't many of those on Fetlife and I was getting waves of personal messages from a real pushy and ignorant Daddy who wanted me to be his plaything even though I explicitly stated I was looking for friends or a Mommy. So, if you sign up for Fetlife, exercise some caution, make a note on your account that you will not reply to certain requests and only respond to those who are in your area, within your age range and who feel similarly about being an ABDL.
You could also try Craigslist. I was skeptical too, but I actually met my current Mommy and another AB friend in the next province over through Craigslist ads. Just make a nice ad, letting people know what you are into, what ABDL means to you and state that you are looking for a nice, platonic playdate. I never thought I would have success through Craigslist, but I did, so the same could happen to you.
Best of luck. If you want to advice on how to go about arranging playdates or meetings with other AB's through Fetlife or Craigslist, feel free to PM me, I'll be happy to help