Policy documents are one of the altmetric sources most crucial for comprehending the interaction between science and policy. In policy documents, the source is usually the institution or organization that published the given policy document. In this study we compare the policy document sources indexed by Altmetric.com and Overton. Altmetric.com is an altmetric aggregator that has been around for over a decade. Overton is a newer database aiming at being the most complete collection of international policy documents. Our findings reveal that Overton covered more policy organizations than Altmetric.com, although the overlap in sources is quite small. This low overlap may suggest that both data aggregators may be using slightly different operationalizations of their notion of “policy documents”, which calls for more transparency on what notion of policy documents and policy organizations are considered by them. Regarding policy organizations indexed by both data aggregators, most policy documents are published by government and non-profit organizations. Future research should delve more into the different policy organizations that are being covered in Overton and Altmetric.com, as well as their geographical distribution.