It isn't convenient to the White Mountains, but Outdoor Gear Exchange in Burlington, VT probably has one of the best selections of both new and used backcountry ski gear in northern New England.
Once you know what you want, it might be worth checking some smaller shops, even ones that don't specialize in tele gear. I've found that sometimes these places can have good deals on backcountry equipment, presumably because they aren't turning over their inventory that fast.