It was the obvious place to look, but nevertheless one of the last places checked when I looked for national aircraft insignia.
Wikipedia.
Be careful, the colours are wrong and some of the insignia are incorrect or flawed. However thay are very large files (2000 pixels the largest size) and therefore great to work with and easy to copy or change.
The older insignia are in their own section underneath the modern/present ones.
http://en.wikipedia.org/wiki/Military_aircraft_insignia
Note that there are several size options for each file.
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