Scientific data is shared in many places. Some makes it into the peer-reviewed articles themselves, or is "Supplemental" data available on the journal's website. Some specialized journals share entire datasets. Some are in repositories for data, which might be at a researcher's institution, might be a shared repository for everyone who works in the same discipline or on the same sort of topic, or a database which sorts and curates data of a particular type across many topics. Keep in mind that human research subjects have a right to privacy of their health information and to information that discloses their identity. This may restrict you to using only data that has already been carefully anonymized, or may require signing data use agreements that will govern how you can use the data.
For help finding data for your work, email firstname.lastname@example.org and we'll help you find the appropriate librarian to assist you!