In reply to tingle:
I seem to remember my Montane Extreme salopettes (like Buffalo, but by Montane) fitting okay over my ski boots.
I haven't used them for a few years, because it hasn't been cold enough. But I do remember they were great for a week of skiing when it was absolutely effing freezing in Les Arcs.
Have just checked Montane's website, and they don't see to do them any more.