Check with large wallpaper stores in your area. The larger wallpaper stores often do have discontinued patterns. You may also be able to get what you need at a discounted price.
Go to online wallpaper stores, such as wallpapermore.com or wallusa.com. Numerous sites online have discontinued wallpaper in stock. Expect to pay extra for shipping.
Search eBay. Do not be discouraged if you do not find the discontinued wallpaper you want the first time you search. New items are listed every day, and the wallpaper for which you are looking may be listed on a new day.
Search craigslist.org. You may be able to find the wallpaper you want in the classifieds on craigslist.org. If not, post the name of the wallpaper you are seeking under the "Wanted" section.
Finally, find sites that will allow you to post your searches along with photos. One good site is thriftyfun.com. If you have a photo of the wallpaper for which you are searching, they will allow you to post it. It also helps if you have a name and/or product number.