Travel clinic. You can go to your local health department clinic for travel immunizations or search the internet for local travel medicine doctors/clinics in your area. They should be able to get that for you as well as the certificate you need for immigration/customs.
Several places. Either there is a travel clinic in your area, or the local health department will either provide this or have information about where you can go to get it. Hopefully you will also be provided with information about other vaccines you need and malaria prophylaxis. For full details see the cdc website for up-to-date information.
Certificate. To obtain an”international cetificate of vaccine or prophylaxis” for yellow fever, get a yellow fever vaccine at a designated vaccination center. The certificate will be valid 10 days after you were vaccinated & is good for 10 years.