For bards, under RAW, musical instruments are allowed as Foci, so I'd allow them to use anything they can use to make music. And, as countless Youtube videos show, that is anything. I'd be incredibly permissive here and allow them to play 'drums' on their goblin rogue's head as a foci.
For druids, it would be anything sourced from nature that is special to them. In my campaign world, Druids draw magic along the weave from the Positive and Negative energy planes to create their spells (with no Gods or other beings involved). It is these forces that are the source of all life and death. As such, the most appropriate things are things that are alive, or once were alive, but stone is considered to be the 'skeleton' of the world as is also capable of being a focus for this magic. I'd allow a druid to use anything made of stone or any biological material as a focus.
However, the only odd focus situation I can recall was in a "you start as a prisoner" adventure where a druid used a rare fungus he took from a cave wall as his focus for a few levels.