In honor of it being Flag Day in the United States tomorrow, here are some resources you might find useful.

Vectors
Wikipedia
SVG format.

JunkMailer (Adobe Exchange)
An awesome set of vector US flags in Illustrator format.

Peirce Clipart Collection
In PDF and WMF formats. Available with shadows on the flag and without.

Photoshop Brushes
SilentMYSTIQUE (DeviantART)
43 high-resolution brushes of various military and patriotic subjects including several flag brushes as you can see. Can be used for any non-commercial or commercial work. He just asks that you “fav” it and leave a link showing how they were used.

Stock Photos
Just a small sample from stock.xchng