The money to finance this lavish lifestyle came from a variety of sources. A special fund was set up in the Progressive Conservative party to collect donations for the First Family. Massive amounts of cash and gifts were brought into the house by the PC machine. But that only brought in a million or two per year. It was far short of what was needed.
So Mulroney set up some special arrangements with lobbying firms that were run by his less reputable old friends. They told him what they needed; he made it happen; the lobbying firm got "success fees" from the companies that got support; and then they funnelled the money back to Brian Mulroney.
One of those firms was Government Consulting International, or GCI. Airbus hired GCI to make sure that crown corporation Air Canada chose Airbus planes over Boeing. Mulroney fired all the Air Canada directors, appointed directors who would vote for Airbus, and voila - Airbus won, and forked over $10 million or more to GCI. Another GCI contract was Thyssen Industrie. Mulroney met repeatedly with Thyssen lobbyist Karlheinz Schreiber to discuss the Bear Head project while Mulroney was prime minister; his ministers signed a memorandum of understanding for the project, which was sufficient for Thyssen to pay Schreiber and then GCI a $4 million success fee, which partly went to Mulroney. None of Airbus or Thyssen or Schreiber knew that the money went to Mulroney - the lobbying firms were run by his good friends and they were extremely discreet. And with Mulroney's power, they were very well compensated.
The beauty of the scheme was that with all the cash washing through 24 Sussex, nobody noticed the bribes. The PC party was providing so much money that everyone assumed all the cash came from them. Mulroney didn't need secret bank accounts because he and his wife spent the money as soon as they got it. Shortly after Mulroney left office he spent a million dollars in cash to renovate his house, but it barely merited a headline - and certainly not a police investigation or tax audit, as it should have. Everyone was used to the Mulroney's predilection for cash. Normal people just couldn't comprehend the insane greediness of the prime minister and his wife.
There would always be rumors, but the only reason the truth ever came to light is that Mulroney cheated one of his backers, the arms dealer Karlheinz Schreiber. Mulroney had played Schreiber for years, telling him that he could make the Bear Head project happen. After he left office Mulroney continued the scam, taking money directly from Schreiber in envelopes stuffed with thousand dollar bills. But one day in 1995 Schreiber learned that Mulroney had cancelled the project in 1990. And the rest is history.
Note: This is speculation.