Corset Story has plenty of plus size corsets, and they are REALLY, REALLY reasonably priced. I havent purchased one myself yet, but I have a few friends who absolutely adore them. They also have waist training corsets. If you dont see a corset you like on the "plus size" page, they actually make a lot of bigger corsets, you just have to look around for designs they are willing to make bigger (it will be in the "size" drop down bar, often with an added fee)
Hope that helps.