I usually wear my 'grandma' undies that my mum bought me when she came to see in me in hospital after my c/s. They were a dollar a pair in target. I'm a size 10 in undies (about a 12 everywhere else after having DD) and she bought me size 14-16. They were awesome after my c/s.
They've now been retired to AF undies. If they get too stained or anything, they can be chucked out. She bought me about a dozen pairs!!