Instructions
Put an ad in the local paper, or a website such as Craigslist. Be specific about the exact calendar you are looking for. Offering a small reward for the location of the calendar may motivate some individuals to respond to your ad. Craigslist allows a user to list an ad for 30 days at a time. Newspaper ads expire much sooner, and you may be required to pay a fee to place an advertisement.
Check listings in eBay and any other auction sites that may exist. Check back often, since new auctions are added all the time. People sell all sorts of things on eBay, including calendars. eBay also offers a service that allows users to list what they are looking for. Other users see this and can make a direct offer.
Visit antique shows and flea markets. Talk to vendors who may be selling older calendars, or other car memorabilia, and ask for advice. Vendors may know enough to point you in the right direction.
Contact GM's customer service division. Since GM published the calendar, it may have some of the calendars available. If not, GM may be able to tell you what companies distributed the calendar.