In the book, it's explicitly said that the mansion is an old De Vil family heirloom.
Though, in the book, Cruella has a husband who actually makes the furs. It's implied she had to marry him to keep the lifestyle she was used to.
EDIT: In the sequel, she's found to be making raincoats out of tough metal instead--after having gotten out of jail. And that's probably the most sane thing about the sequel.