The Praça da República is located smack downtown Sao Paulo and has a Metro station (subway) located on, and named after it. During the business week, it can be a spooky experience to visit, but on Sundays you will find that the city and the park itself wind down to a more human pace.
On its premises you will find an assorted variety of kiosks that sell all types of souvenirs like fancy rocks and beautiful birds carved in stone. Also some really nice paintings sold by the very people that created them.
Considering the high cost of living in Sao Paulo, the prices are not that bad, and you can buy very descent paintings for under 50 bucks.