Several of my friends are getting to the postpartum oh-shit-I'm-no-longer-pregnant-but-nothing-fits stage. Well, guess what? Nothing should fit. Anyone who says differently is delusional or had a very unique experience. You just grew a human with your body! I didn't fit back into my pre-pregnancy jeans until around 11 months postpartum and I'd say I'm pretty lucky. And it's not just your tummy that's different, your body is holding onto fat stores all over (this article explains why). Here's my advice (and I don't say this very often): buy new stuff. It's the only solution. You need real clothes for the size you are.
You don't have to go all out (and you probably don't have the time). Budget a small amount. As I mentioned in this post about transitional dressing, this is one time it's ok to buy clothes that are inexpensive. See this post for nursing basics and why you want to buy things that you don't care about getting ruined.
When I started looking for a few examples for this post, I got a little jealous. I honestly feel like there are better options now than there were for me last summer! At minimum, you should have 1 or 2 pairs of jeans or other bottoms, a few fun non-clingy tops, a couple of decent basic tees in neutrals to dress up with jewelry and scarves, and a couple of nursing friendly dresses. Leggings can still work when you add one of these tunics for a little polish.
What are you wearing for postpartum?