Belém's relationship with açaí is older and more serious than anywhere else. The berry grows wild in the estuary floodplains and arrives at Ver-o-Peso by boat each morning, already pulped and packed into plastic tubs. The local version is dense and unsweetened — the color of a bruise, with an earthy, faintly fermented flavor that takes some getting used to.
At the stalls around Ver-o-Peso, açaí is served in a bowl and eaten with fried fish, dried shrimp, or manioc flour. Ask for it "natural" if you want it plain; some vendors add sugar without being asked. A small bowl costs almost nothing and will hold you for several hours.
If you want a more comfortable setting, several restaurants near the market serve açaí properly alongside a full Amazonian meal. But for the real experience, the plastic stool at the market stall is the correct choice.