If you are near Lower Bucks, I think I've seen them at both the Salvation Army and Goodwill stores near Oxford Valley mall. Also check out the big weekend consignment sales. They always have tons. Lil' Angels consignment is holding their sale this weekend in NE Philly...and Sunday is half-price day.
Re: Thrift Stores with Maternity Clothes?
Website: https://conshyconsignment.com/