How can one find a grave of a person who is buried in Houston?---any government agencies or web sites?

If you know when they died, you may want to go to the library in Houston and look through old newspapers. Sometimes they put them on microfilm. There should be a death announcement in the newspaper that tells about funeral arrangements. You could also call the local cemetary or person who is in charge of it and ask if they have a plot with the person's name on it.