Shauna from Shwin & Shwin shares a free pattern for her Stars & Stripes Baby Romper. The romper is a variation on her Summer Breeze dress (free pattern, linked in her post). She uses the dress bodice but replaces the full skirt with the romper pants. The romper she made was a 4th of July outfit, but it doesn’t have to be a holiday outfit. Her pattern is for a size 3-6 month. If you need a different size, it should be fairly easy to adjust it. Go to her blog for the pattern and tutorial.
[photo from Shwin & Shwin]