Online research methods are increasingly used in conservation science to monitor and collect data on online wildlife trade. Shark‐like rays (Rhinopristiformes) are among the most threatened vertebrates, prized for their high‐value fins while other uses for their derivatives are largely unrecognized in the literature. The bowmouth guitarfish (Rhina ancylostomus) is characterized by ridges of enlarged thorns, sometimes sold as amulets. Listed on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix II, the thorn‐market has been ignored and undocumented. Using systematic, retrospective online searches, we recorded 977 bowmouth‐derivatives for sale. In our global and multi‐lingual scope, 97% of products were offered from Thailand, despite prohibited trade. 98% of all products were thorns (USD3‐300) and sold alongside high‐profile CITES‐listed taxa (e.g., tiger claws). Consistent advertisements from 2012 revealed a previously unquantified trade of an often‐neglected marketplace in elasmobranch research. Our findings offer insight into the drivers of the thorn trade, potentially rooted in cultural beliefs. Our accessible research methods can be applied internationally and across species, providing quantitative and qualitative insights on aspects of the wildlife trade of a critically endangered fish. Continued monitoring of online markets, like this study, is necessary to fill critical gaps for informed policy and species‐specific management.