How old is this house? If its a newer home and in the city limits the Dallas Planning and Zoning Department may have a copy. If its not new, you can find the builder many times by searching the title of the property back to the origin.You can do that at the recorders office yourself or pay a title company to do it for you. The builder may still have the plans he may not. If its a vintage home and the builder is no longer alive, a local draftsman or architect can re draw it for you. Remember many plans are copywritten.
If you merely want the "footprint" go to the assessors office and ask for a copy from the assessor's file. They will charge you a nominal fee for a copy.
Good Luck... more