Skirt sports has one or two skirts in XXL-- which as a size 16-18, I find to fit, but if I have anything in the pockets, it starts to fall off. The XL works, but the skirt is a little tighter around my butt. I like the looser skirt, so I put up with it.
The coverage ranges from runderwear to compression shorts (which have a neat pocket that fits my iPhone) to capris. I LOVE this company. They've got great customer service and are pretty flexible-- if you don't know your size and want to try them on, they're really good about sending over ones for you to try if there's not a store that sells their stuff local to you.
Also, I still have EVERY skirt I've ever bought from 2006 on-- I only run races in their skirts-- all in great condition, and they've been put through the exercise ringer.