Squadron is a great source. Also, try:
http://greatmodels.com/
http://www.rollmodels.com/
Try some of the manufacturers' sites:
http://aeromaster.com/
http://www.tallyho.cz./
http://www.twobobs.net/
http://www.superscale.com/
And there are a few regulars on eBay who have fixed price decals, one of my faves is
dtazzygirl.
Fade to Black...