You'll find this in Undead Crypt. From the Undead Ditch bonfire, move through the next two rooms full of stones. In the large, empty room, interact with the wall on the right to reveal a secret chamber. Watch out for the two wall ghosts in here, and open the chest for the Avelyn crossbow.
Bow of Want
Can be traded for Nashandra's Soul with Ornifex near the Lower Brighstone Cove bonfire in Brighstone Cove Tseldora.
Can be traded for the Mytha, Dragonrider Soul with Straid of Olaphis at the Straid's Cell bonfire in The Lost Bastille.
Heavy Crossbow +3
This is found at the beginning of Earthen Peak. Past the two rooms full of pots is this room with a flooded poison floor and several sets of stairs. Note the ledge across the gap. It takes a very careful leap from the nearby stairs to reach it without rolling off, but if you make it, you can follow that passageway to a chest containing this weapon.
On your way up Drangleic Castle, look for an easily missed ladder just before Nashandra's chamber. Climb down, then follow the ledges to a chest containing this bow.
Can be traded for the Executioner's Chariot Soul with Straid of Olaphis at the Straid's Cell bonfire in The Lost Bastille.