Welcome to the Forums.
Snow Tracs come up for sale from time to time but you have to keep checking. As we approach winter its more likely to see some for sale in the fall than during the summer.
Keep your eyes peeled, check here frequently, try to network with people who may hear about them. Often they are listed on a local Craigslist, or Ebay and then get re-posted here. But vigilance and persistence is your friend when searching for a Snow Trac. It might also help if you listed in which part of North America you are located, US/Canada, East/West. As many sellers will only deal with people in their region.