Postpartum belly bands. From reality star-brands to medical grade ones...there is a bunch out there.
The point of a belly band is to help aid with ab and lower back support, and help pulling the belly back in after 9 months of carrying another human being. Your belly has grown, your hips have gown and all of a sudden you're still walking around like the scarecrow in The Wizard of Oz from the relaxin still floating about in your body and just need a little support.
The one I like most (and I've tried quite a few of them between my two postpartum rodeos) is the Upspring brand belly band. It's torso-length, breathable, can help with hip compression, but the best part is the wings they include to help with support needed around the waist for a comfortable and snug fit.
I've seen both charcoal and nude colors and it's thin enough that it easily tucks into pants without bulkiness so wearing under clothes isn't a problem if you need to face the world soon after giving birth.
I'm just starting to think about working out again after giving birth to my second little guy and although I haven't worn my belly band for several months, I'm considering wearing this during my runs just for peace of mind and support. I'll let you know how that goes :)