Otherwise, look into the La Sportiva Lhotse GTX ($325; www.sportiva.com). It comes in women's sizes, so that's a good start. And although it's not a true "plastic" boot, it's a plastic/leather hybrid that is both waterproof (Gore-Tex liner) and insulated. And they're good, sturdy boots, with thick leather, full rubber rands, and full steel shanks.
Otherwise, try the usual tricks when working to make boots fit. Try an after-market footbed for high-volume shoes, as that may soak up some extra space. And, you can sometimes find heel inserts that wrap around the heel and reduce the volume there, as that's a particular trouble spot for women trying to wear men's shoes.
Hope that helps!