I’ve mentioned before about using Find-a-Grave for tracking down your Civil War ancestor. You can read that post here. There are other sites, if you don’t have any luck with that source.
Try the Billion Graves site. It functions quite a bit like Find-a-Grave with volunteers submitting photographs of the gravestones in the cemeteries across the U.S. and around the world. Put in your ancestor’s name and if you have it, the year for birth and death.
There’s an advanced search where you can narrow the search down by state and county. If you already have an idea of where the grave is, this works fine. If not, the site recommends not searching too specifically. The ancestor may surprise you by being buried some distance from where you expect them to be.