Here is the full text of the rules (from http://5e.d20srd.org/srd/equipment/equipment.htm):
Arcane Focus. An arcane focus is a special element: an orb, a crystal, a rod, a specially constructed staff, a wooden wand similar to a wand or a similar element, designed to channel the power of arcane spells. A sorcerer, sorcerer or wizard can use that element as a spell focus.
Druidic approach. A druidic focus can be a sprig of mistletoe or holly, a wand or scepter made of yew or other special wood, a cane extracted from a living tree, or a totemic object that incorporates feathers, hair, bones and teeth of sacred animals. A druid can use an object as a spellcasting spotlight.
A super-literal reading of the rules would say: an arcane focus is "a wand designed to channel the power of arcane spells." A druidic focus is "a yew wand or other special wood".
It sounds as if you could look for someone who can make arcane foci and ask them to make an arcane focus from a piece of shingle wood. Then, because it is made of yew wood, it will also work for druidic spells.