For that one I would look for a book author of a recently published book on motherhood. Mags love pairing new books with their stories and the authors are eager for a plug so make good interviews.
To find one, head to Amazon.com (or the like) and search for books on parenting that will be released in the next 6 months or so. Then contact the publisher for the author's agent - typically they'll send you a copy of the book and hook you up with an interview.
For example on this I searched for 'motherhood' on amazon, then sorted the results by publication date. Next I would go through the books and choose ones that had an AP focus and then make contact with those authors (I'm a big believer in subversive journalism
I found about four books that look like they would either address the issue or that the author would at least be able to give intelligent comments about it.
For a query - you don't need to have done the interview yet, just give some examples of who you would interview (this shows you know how to find them.)
Beyond books, you can do a google search for current studies on your topics, then contact the study authors. For example, you may have learned from anecdotes that women who don't get breaks are more stressed - so you could google "study stress motherhood" and see what you get. For more general stuff I contact the media department of universities and ask for specific types of experts. Most universities have listings of the staff who talk to journalists - as well as what their areas of expertise are.
Another source is https://profnet.prnewswire.com/
It takes a bit to get the hang of and is better when you have assigned stories - but it sends out your request to qualified experts for free. So for example, if I'm writing a story on why it is important to talk to your newborn and want comments from a range of experts, I'll fill out the form saying I need experts who can tell me why it is important to talk to babies and within a few hours I'll have a dozen responses to wade through.
Personally, unless I'm writing for a regional mag, I typically avoid local sources and just save those for emergencies. You'll run out of them too quickly and most mags like you to cover as broad a geographic area as possible.
Does this help?