Motherhood Maternity. You can also find them at Babies R Us. The ones with a matching robe are $35 or $40. There are some without the gown for $20 or $25.
The gown was nice for in the hospital and the first couple weeks at home, but I didn't wear mine after that. So, don't fret if you can't find one. I was quite shocked at their prices, seeing as they are all covered in those horrible floral designs. Seems like Target might have had one with a little more modern look.
Stretchy tank tops (I loved the Old Navy ribbed ones) and some comfy pajama bottoms (stretchy, cotton ones big enough for the post-baby belly) work just fine too.