Where can we find malware source code? This question is motivated by a real need: there is a dearth of malware source code, which impedes various types of security research. Our work is driven by the following insight: public archives, like GitHub, have a surprising number of malware repositories. Capitalizing on this opportunity, we propose, SourceFinder, a supervised-learning approach to identify repositories of malware source code efficiently. We evaluate and apply our approach using 97K repositories from GitHub. First, we show that our approach identifies malware repositories with 89% precision and 86% recall using a labeled dataset. Second, we use SourceFinder to identify 7504 malware source code repositories, which arguably constitutes the largest malware source code database. Finally, we study the fundamental properties and trends of the malware repositories and their authors. The number of such repositories appears to be growing by an order of magnitude every 4 years, and 18 malware authors seem to be "professionals" with well-established online reputation. We argue that our approach and our large repository of malware source code can be a catalyst for research studies, which are currently not possible.