I like the guys at Gold Medal Sports (goldski.com). The shop is by the ski area, but kind of hidden. If you come from the Durango direction stay on the highway and go about 100 yards north past the big entrance to Purgatory (DMR) and turn left, park, and walk down the snowy alley behind the buiding. The shop is in the back of the old Lodge at Purgatory, and you can enter from the front or the alley in the back. They've been around for at least 25 years and know their stuff. They are a Dalbello boot dealer, but also sell several other brands. Since they are right at the area you can go in for a mid-day tweak on the fit if you need it.
IMO the only other really pro boot fitters in Durango are at Pine Needle Mountaineering right downtown between 8th & 9th on Main, they only sell AT and tele boots but can fit just about anything.