Spirit carries plus sized costumes. Try before you buy tho, as some of them lie about what size they really are.
Lots of luck - I know how hard it is to find good plus-sized costumes, being one myself. I wear an 18, but I'm big busted, so I have difficulty finding stuff that fits perfectly unless I alter it.
(I'm good at sewing, so I make most of my own costumes, but I splurge sometimes on the more elaborate stuff cause I get lazy)