The best fat pants are pregnancy pants. I searched our local op shop for bigger board shorts so I could still swim regularly. The brown pair fit me that day, and I went for a swim immediately (the day before I became properly sick) and I figure the green pair (which have a drawstring, and which were most definitely part of the men’s section) will still fit me if I’m eight months pregnant with triplets.
Since it’s an charity secondhand clothing shop, the two pairs of boardies and a pack of shampoo cost $10 altogether.