Many! In biology and medicine, pubmed is the most common ( https://pubmed.ncbi.nlm.nih.gov/ https://pubmed.ncbi.nlm.nih.gov/ ) . For general purpose you can u

Many! In biology and medicine, pubmed is the most common ( https://pubmed.ncbi.nlm.nih.gov/ https://pubmed.ncbi.nlm.nih.gov/ ) . For general purpose you can use google scholar ( https://scholar.google.com/ https://scholar.google.com/ ). When you find an interesting article, both these tools will allow you to both seach backwards for articles that it cites, and search forwards for articles that cite it in turn. Repeated application can quickly help you expand the pool of potentially relevant articles (especially once you surface review articles in the citation chain, which can help tie things together and/or let you jump across to associated topics) . Finally, both of these tools yield you a DOI which you can plug into scihub. You can also install the extension "Sci-Hub X Now!" to do this last bit for you. ( https://github.com/gchenfc/sci-hub-now https://github.com/gchenfc/sci-hub-now )