"Pac boots" have a rubber/leather/nylon outer shell and a removable liner made of felt and/or other insulating materials. Most of the Sorel, Kamik, Baffin, etc. brands are built this way. Some brands of mountaineering boots use this arrangement also, out of practical necessity.

As Todd_W suggested, it's a challenge to find pac boots that have the rigidity and support necessary for long-distance walking or snowshoeing. Most are "snowmobile boots" that are soft and floppy like bedroom slippers.

If you want a genuine hiker with a removable liner, it's not going to be cheap. I have seen some in serious outdoor stores, starting at approx. $130 CAD and going up from there. Mountain Equipment Co-op (mec.ca) often has some likely candidates.

I have also found that pac boots that are CSA-approved (green triangle, safety toe and footbed) are more rigid. Synthetic toes and puncture-resistant footbeds don't suck the heat away as fast as old-school steel ones.

BTW, sometimes you can extend the temperature range of a standard boot by wearing neoprene kayaking booties over a wool sock. My initial trials have been quite successful, but I've never tried it overnight.