The Civil War, fought between April 1861 to April 1865, so, any cover mailed during this time frame basically qualifies as a Civil War cover. So, do you have any plan as how to specifically qualify an item, a stamp or a cover, for your specialized collection?
There are a number of stamps and postal stationery items depicting scenes and/or personages from the Civil War. Most of this material is reasonably priced. The ATA may have a Civil War specific list available.