I second just getting wind resistant tights. More comfortable and versatile, and less bulky, than insulation + windpants. I think it's tough to find windpants that aren't baggy and flappy, and that's a problem on the bike.

I've worn Sugoi Subzero tights comfortably from 10 degrees to about 30 degrees on the bike. Craft Storm Tights, with or without longjohns underneath, are also very warm (I've worn them below 10 degrees with longjohns).

Lots of people love their PI Amphibs. I haven't tried them, but they seem to get raves.

If you must have separate wind pants, I might start out by looking at products made for XC-skiing. They often cross over well to cold-weather biking.