You want to try Farmacias Sucre out here for vaccines (I think they were the only ones I could find who did the yellow fever one), although maybe the best way to take it would be seeing a doctor to get a prescription after checking your antibody levels. Most pharmacies here have a general practitioner in there who maybe even owns the place for consultation... just look for the person in the white jacket. Chances are their English is fine. Also looks like Clinica Biblica has it for only about 88$, they're alright: