First (as a former Coastie) I have to commend you on your knowledge that the United States Coast Guard is actually a branch of the military. Seems almost everyone gets that wrong, even the media.
The above links that Lisa posted look like some of the better ones you will find you can also go to the US Coast Guard web page. http://www.uscg.mil/USCG.shtm
If you look around you can find a lot of pictures on there that may work well for an edible image. If you look on the right side you will see two links 1 for Units and 1 for Aircraft and Cutters. If you are making a cake for a person in a specific unit you can look there and possible find a unit patch or something that will work.