Watchtower accountants and lawyers go to great lengths to keep the finances secret. But there are a few tidbits that have been made public. The following article from Watchtower gives a glimpse into the finances in the 1930s. I suspect large donations and inheritances make up a large chunk of their income. And Watchtower is heavily invested in real estate, holding title to about 40,000 properties, mainly KHs and property left in wills. But good luck finding documentation. We can only speculate on much of their finances, as I have done here. A recent leaked video of their accountants mentions that they are trying to sell 3,000 of their properties.
http://watchtowerdocuments.org/heir-to-coca-cola-fortune-was-a-jehovahs-witness/