I was following a thread on The Railwire where the photos included a car traveling the layout that had had a flag added to the side of a car. (The thread wasn’t about the flag.) The flag looked like it had been cut out of a photo of a flag waving in the breeze. Personally, I didn’t like it—not that it matters; it’s what the owner of the car likes that matters.
That got me to thinking about flags and how to make them in N scale.
I did some quick math (using my N-Scale/Prototype Conversion Calculator) and determined that an N scale 3’ × 5’ flag like people fly in front of their houses would be 0.225” × 0.375” (a little under 1/4” × 3/8”). That’s tiny! You need a printer with high resolution to print decals yourself.
At 600 dots per inch (dpi), 0.375” wide flags would be 225 pixels wide and have this amount of detail:

3/8” wide flags of Canada, United States, and Mexico at 600 dpi.
This level of detail isn’t great for a larger image, but when you consider how tiny it would be in N scale it should be acceptable. Of course, printing at 1200 dpi would produce an even sharper image.
So where can you find flags? Wikipedia to the rescue! Wikipedia has lots of flags, both current flags and historical flags. You can find Canadian flags here, United States flags here, and Mexican flags here. If you’re not looking for national flags, look up the state or province and you’ll almost always find its flag on the page. You can also look up historical flags.
Here are the current national flags of Canada, United States, and Mexico. Right-click on the image and select “Save image as…” to save the file to your local computer. (The actual wording of “Save image as…” might be slightly different depending on which browser you use.)
Note: Different countries’ flags have different height-to-width ratios, so the images above don’t line up exactly. However, the ratios are accurate.
The flag images above are in SVG (scalable vector graphics) format. A vector image is one that can be scaled up or down with no loss of resolution, in other words, it won’t pixelate the way raster images (JPG, PNG, GIF, TIF, etc.) do. You might need to convert the SVG image to another vector format like PDF, EPS, AI, etc. before printing. I use Inkscape for most of my vector image work. It’s free and it works with many types of vector images.
Flag Trivia
The United States had a 49-star flag when I was born in December 1959.