Try Dallas Co, TX GenWeb site, some GenWeb counties are putting their cemeteries online. If you can find death date for William, you can obtain a copy of his death certificate, which should show the names and birth locations of his parents.
Have you checked census records, beginning with 1850, after he is married and having children, then follow through and pick up names of any other children. The 1900 asks some questions not on other censuses, and one is month and year of birth of everyone in the household. You will at least pick up that on William. Beginning with 1880, birth location of parents of everyone in the household will be listed. On the 1900 it will also ask how many children a woman has had, and how many living at the time of the census.