C.M.
I believe Oshkosh also has an online store and outlet stores. Have you looked at their stuff? I know Sonoma pants usually have the little elastic sizing strips that you can button so they might be good for you to try. Buy 'em bigger, size the waist with the elastic and maybe just get some iron on hemming tape for the bottoms. That way its temporary and you can make them longer as needed.
Oh and I find overalls are nice for cloth diaper babies. They're a bit roomier in the butt. My son usually wears them when we're just out and about, you can get them in cords, khaki and denim so they can look dressy and casual as needed. We usually go for the lined ones since we're in a pretty cold area for winters.