São Paulo (capital) has a very famous street called Rua Teodoro Sampaio which is known for having almost all the music stores in São Paulo.
Its very easy to locate a good store there, you just have to look for a big store which looks like it's trustworthy.
There are lots of good stores and some include:
Making choices on what instrument you want to buy is not that complicated, you can go to the store and ask for professional counseling (all the workers there are semi-pros or pros in music).
The process is simple, you tell them the range of price you want to pay, they show a various instruments that suit your needs in that specific range of prices, you choose one, test it, see if you match that instrument ( by match I mean that if you feel comfortable playing it) then buy it and enjoy.
So whenever you want to buy a musical instrument check out Rua Teodoro Sampaio which is next to Oscar Freire in the Bairro Pinheiros