The fireplace appears to always spawn, but the logs may not. If the logs do not spawn in the fireplace a player will not receive the shinbone. If this happens the player must reload the game by clicking Leave Game, then Resume Game.
Can be found in Adventure Mode, which appears to have two fireplaces in 2.4.1. Bone was found in the lower fireplace near the entrance to the manor but may have been in both prior to the bone being taken.
Work from Act 1:9 ("The Imprisoned Angel") or beyond (while finishing "Trailing the Coven", this can still be performed. To get there take the waypoint to Leoric's Manor (last one if a player choose The Imprisoned Angel quest). When inside, just head back on the stairs and find a large chamber with the fireplace at the end on the right side of the map. The entrance to the chamber is quite small, but it is on the far right side of the Manor.
There will be a checkpoint that will allow a player to leave the game and rejoin it in the mansion to quickly farm the item.