S.K.
Here is a link to some on amazon. But I know I bought some white cotton ones at walmart when my daughter was a baby. They were called "rhumba pants" and they were by the infant socks/tights. But that was 6 years ago.
http://www.amazon.com/Ema-Jane-Ruffled-Diaper-Bloomer/dp/...
You could also check out etsy.com and search for "infant bloomers" or "ruffle butts" as I used to call them as a child. Hope you find something you like.