The 1900 census indicates he and Lizzie were married 6 years. I suggest getting the marriage license. They probably got it at the Washington County Register of Wills. It may have data on parents.
If you know when and where Thomas died I suggest getting his death certificate and obituary. If in Pennsylvania the PA Dept of Health would have the certificate. Check with the Washington County Historical Society about the obit and burial place if you don't already know it.