FindAGrave.com is our go-to source for cemetery records, but it's a long way from having every burial. There are other websites that can be used to search for relatives that may be missing from FindAGrave. Here are just a few, but you should try to find other sources that may be useful for your research:
National - The National Cemetery Administration of the U.S. Department of Veterans Affairs provides a Nationwide Gravesite Locator. A search for my father-in-law shows that he is buried at section 1, site 776 of the National Cemetery of the Alleghenies in Bridgeville, Allegheny County, Pennsylvania. It also provides his rank, branch of service, birth date, and death date.
Regional - For the Pittsburgh area, the Catholic Cemeteries Association provides a Search Burial Records page for their 14 cemeteries: All Saints, Calvary, Christ Our Redeemer, Good Shepherd in Monroeville, Holy Savior in Gibsonia, Holy Souls in Coraopolis, Mt. Carmel in Verona, Our Lady of Hope in Tarentum, Queen of Heaven in McMurray, Resurrection in Moon Township, Sacred Heart in Monongahela, St. Mary, St. Joseph in North Versailles, and St. Stanislaus.
Both sets of my grandparents are in Queen of Heaven Cemetery, and I also have Stern cousins in Calvary Cemetery. All of them can be found by searching the Catholic Cemeteries Association, but only half of them are in FindAGrave. The Catholic Cemetery site seems to be up-to-date since a family burial from last September is in the database.
Individual Cemeteries - My 2nd-great aunt and her husband, Clara & Joseph Carr, are buried in Grandview Cemetery in Johnstown, Pennsylvania. Their burials weren't in FindAGrave until I added them. Grandview Cemetery provides an Interment Lookup page, as do many individual cemeteries, which could be helpful to your research. In addition to interment date, Grandview Cemetery also provides place of birth and age at death.
For each of these examples, no gravestone photographs are provided. However, they do provide section and lot information which can then be included in a photo request for FindAGrave's great volunteers. Including this detail will increase the chance that the grave can be found and your photo request will be fulfilled.