How do I replace a missing exterior wood shingle?
You should be able to get a replacement shingle at a lumber yard or home center. They may sell them solo or you may have to buy a bundle. Looking at the photo it is hard to tell the style but in my locale they come split or sawn. They are installed either sanded or rough side out. Pick a shingle with the best appearance match and wider than needed. Trim it to fit with a utility knife including the angle cut at the top. Ideally you want the top of shingle to be under the trim. If the old shingle fell off rather than broke slide it under the trim. Stain both sides and the edges of the replacement shingle. To attach the shingle use exterior adhesive or drill a couple of small pilot holes in the face of the shingle. Use 4d stainless steel nails to attach it to the sheathing. If you were unable to slide the shingle under the trim put a bead of chaulking  where the shingle meets the trim.