I did the same thing; built posterboard mockup with taped on pictures before cutting panel.
I don't remember specifically where I got all the pictures but in general checked the manufacturer web site for product I needed a picture for and then searched retailer sites if needed. Can then crop and edit size as needed to actual size (doesn't really matter if that distorts the image as long as the size is right).
mockup panel looked like:
final panel result:
Other panel in progress pics at:
http://rv6aproject.ckhand.com/panelA...Electrical.htm
Chris
RV-6A, almost ready for inspection & first flight
Mountain View, CA