The default ones are stored (you might say hidden) inside the FileMaker Application. On Mac, you can see them if you select 'show package contents' from the contextual menu when you right-click on the FMP14 application. They're in the subfolder Contents/Resources/. Apparently, it's a bunch of small TIFFs. On Windows, it's probably something similar.
I wouldn't recommend messing with the contents of the application itself, though. Apart from the risk of simply breaking it, I also wouldn't be surprised (haven't checked) if your License Agreement doesn't allow you to alter the application in the first place.
Custom icons must be stored inside a file, as they travel with it. There is no easy way I know of to access them.
I was going to change the background color of an icon and then insert it as custom icon. Or insert a new one but my .png or .jpg were garbled.
I'll try with a .tiff and then I'll let you know.
Meanwhile thanks for the information.
I found them.
On windows the icons are stored ( as png ) into a .dll file ( Extension/English/FMRSRC.dll ), while their localized name is stored into a .dls file ( Extension/English/FMStrs.dls ) that can be viewed changing the extension to .fmp12 ( StarterSolutionIcons Table )
This is the FmrsrcDLL PNG 404 icon: