There are some different vendors out there that will carry them.
Here is one of them:
http://www.taillightking.com/60-66ChevyTruck.htmThe correct marker for your truck is a Yankee 77. Part number 30076.
Click on the picture for a larger view.
They made about 2 or 3 different paint colors for the interior. A good auto paint store should have the original paint chips, numbers and mixing code availabe.
Bill