Veneer's normally a standard thickness, which is around 0.8mm. I've got mine online. Marquetry packs are good for smallish-sized bits of veneer. You may find some in a general DIY store, but probably easier in a more specialist timber store, or even an art/craft store.

The base is the most important part of the pocket as it sets the neck angle. You can do some light sanding to remove any rough or high spots, but you don't want to sand much more than that as you don't want to alter the angle, (unless it's obviously very wrong).