Beco do Batman, in Vila Madalena is the obvious place to start. Quite a safe location. Downtown will be a bit more iffy so better to be in a group.
I quickly did a Google search for "tour graffiti sao paulo" and the first results seemed to be exactly what you're after...
https://www.concretejunglestour.com/...-graffiti-tour
https://urbanbikesp.com/bike-tours/street-art-graffiti/
https://theculturetrip.com/south-ame...-in-sao-paulo/