During carnival, the blocos parade on the street! Sometimes you can, literally, turn a corner and find yourself inside a bloco.
Anyway, there is an official calendar you can consult:
The most famous ones are very crowded, though, and it's worth a try, but you should focused on the smaller ones - ask locals for advice on the day because most blocos don't have a official time or place to start or finish.
Some famous and traditional ones: Escravos da Mauá; Banda de Ipanema; Cordão da Bola Preta; Imprensa que eu Gamo; Suvaco de Cristo; Céu na Terra; Boitatá; Boitolo (and many more)
Some less popular but full of locals: Maracangalha; Baianada; Bloco Cru; Gigantes da Lira; Bagunça Meu Coreto; Exalta Rei (and many more too!)
Happy folia :) about 5 years ago