Since I'm still biking about 12 miles a day, and since almost all of that is on unplowed streets, I've been looking for a bit more traction. Bike stores here actually sell factory-made studded tires for commuter and mountain bikes, but as you can see, they ain't cheap.
I've made my own pair, and while I don't have a ton of mileage on them yet, they are working very well. And they cost $2.85. Total.
A great Missoula resource is Free Cycles. Free Cycles has a nice, clean, well-stocked shop and people who know what they are doing. They collected discarded, scavenged, and donated gear, spruce it up, and offer it and the workspace for free or by donation. Using the advice of Bob, the shop chief, I got four mountain bike tires, two which were about 2 inches wide and with good tread, and two which were about 1.5 inches wide and with well-worn tread. I then went to Ace and bought a box of 185 tiny sharp screws. I then sat on the kitchen floor for about two hours last weekend and screwed the screws into the good tires, then shoved the older thinner tires inside the good tires, then mounted the double-tire set onto the rim and inserted the tube. The outer tire has good tread about about 80 sharp-as-hell screws sticking out; the inner tire acts as a buffer to protect the screw heads from the tubes.
I can report so far that he upside is that I have very good traction. The downside is that I have bike tires that weigh about 10 pounds each and a LOT of rolling resistance.
We'll see how long this works.