I found some at Jc Penney. But I couldn't justify $30 for a pair of shorts I would wear for one summer only. So I wore just bigger sizes, I bought regular non maternity jeans and shorts as well as bigger sweaters and t-shirts. I went from my normal 22 to a 28. I had two pregnancies one through winter and one through summer and did not buy one maternity item. But, I only gained 12 pounds with my first pregnancy and 14 with my second (which my doctor was ok with since I was already over weight)
I wish I had some better advice but I searched and searched and couldn't find anything, so just so you know it's not just you who can't find stuff. Its' something I think that needs addressed by designers.
Best of luck to your neice and congrats on becoming a great Auntie