There is much we do not know about the early development of double entry bookkeeping. What, for example, caused it to be used by sufficient merchants for it to be formally taught to their sons in Northern Italy before anyone had apparently written anything about it? And, what did Pacioli use as the source for his 1494 treatise, the earliest known detailed written description of the method, something that has challenged researchers for at least the past 130 years? Discovering Pacioli's sources could broaden our knowledge of the Renaissance roots of accounting and of its early role and place in business practice; may provide some insights into the reasons for the emergence of double entry bookkeeping; and may give us further insight into the early instruction of double entry bookkeeping. But, previous attempts to find his sources have failed. Making use of hitherto overlooked information, this paper identifies two periods for which knowledge of Pacioli’s whereabouts would indicate where to focus any search for his sources and suggests where to initiate the search.