For advisories before and during travel, the strongest free resources are national public health authorities: several governments run excellent travel health websites with country-by-country pages covering recommended vaccines, malaria maps, and current outbreak notices, and the World Health Organization publishes international guidance. Your own country's foreign ministry travel advisories add the security and practical layer. These update continuously, which is exactly what you want over static guidebook advice.
On the ground, travel clinics exist in most major cities: look for travel medicine departments at large private and international hospitals, vaccination centers run by public health authorities, which are often the designated yellow fever centers, and standalone travel clinics in expat-heavy areas. They handle pre-trip consultations if you're moving onward, boosters, malaria prescriptions, and post-travel checks if something followed you home. Embassies typically publish lists of recommended local doctors and clinics on their websites, which quietly solves the which-clinic-is-reliable question.