The quote you are looking for comes in Act V scene 1 just after Romeo has sent Balthasar away to hire some horses. Romeo has clearly devised his plan to die with Juliet, his wife, that very night, and needs some means of ending his life so that he can commit suicide. He remembers seeing an Apothecary that he had seen thereabouts, and he describes him as follows:
I do remember an apothecary,
And hereabouts 'a dwells, which late I noted
In tattered weeds, with overwhelming brows,
Culling of simples. Meager were his looks,
Sharp misery had worn him to the hones;
And in his needy shop a tortoise hung,
An alligator stuffed, and other skins
Of ill-shaped fishes...
Key elements in the description of the Apothecary therefore are the poor state of his appearance and clothing and the way that he carried with him many different kinds of herbs. He is obviously very thin and gaunt as his looks were "Meager" and his "misery" was evident. Romeo therefore concludes that this would be the man to approach to buy his poison.