Are you looking for prepaints or are you happy with unpainted ones as well?
I don't really know of any figs that could serve as bloatmages, but there are some minis available that you could convert into something that looks the part.
Heroclix has some 'obese' miniatures, try looking for King Pin, Mr Hyde, the Blob or Big Bertha. These minis are still readily available on the second-hand market. If you are looking for a female fig, Horrorclix alos released a Big Bertha in the past, although she might be a bit harder to come by. These minis would all benefit greatly from personal conversion.
Lead adventures Brueghelburg (Ratnik) offers some 'heavier' medieval miniatures. The archivist and merchant set holds two minis that approach a bloatmage's build. I used the archivist for Togomor in my Curse of the Crimson Throne campaign. In the adventure Togomor is supposed to be a bloatmage, although I did not make him one in my campagin, but I did keep him somewhat overweight.