Robert, shown below are Nikon camera pixels to megapixels. You need to look at images no smaller than 4288×2848 (this is a 12 megapixel image) for your banner, the larger the better of course. The quality of the photo is what matters the most. 3008×2000 = 6.0 megapixels, APS-C (23.7×15.5 mm) 4288×2848 = 12.3 megapixels, APS-C (23.6×15.8 mm) 4928×3280 = 16.2 megapixels, Full frame (36×23.9 mm) 6000×4000 = 24.1 megapixels, APS-C (23.5×15.6 mm) 6016×4016 = 24.3 megapixels, Full frame (35.9×24 mm) 7360×4912 = 36.3 megapixels, Full frame (35.9×24 mm)
There are many stock sites that sell large megapixel images but the general rule is the higher the resolution the higher the price you pay. Just Google for, hi res royalty free stock photos.
I have a D300, D800, and an old manual Nikon FE, my first ever SLR.
Robert, any image around 10-12 megapixels interpolated using a fractal resizing program would be of benefit to you, Before my D800 I used to enlarge 12 megapixel Images to 30+ megapixels with great output. At 3m in height when viewed at a distance the images will look good. The quality will depend on the original pictures though. Most banners that large are only printed at around 120dpi
Hope that helps you.