From what I know, this achievement can be acquired as early as in act 2, in the shadowlands. It's acquirable north of the Shadowed Battlefield fast travel point at X:40 and Y:36, below one of the twisting branch bridges that lead north.
If you cannot find it there, or made it past act 2, more of them can be encountered and acquired in a secret little room that hidden by some boards in the pantry room inside the house of grief at Baldur's Gate.
Do beware however, this room can only be access have you finished the fight against viconia. Inside, you should find plentiful of flowers to gift to lovely shadowheart!
To gift her the flower, all you need to do is speak to her and there should be a dialogue option, "Remember when you told me night orchids were your favourite flower?" And she'll be very happy in no time!