Those jackets are referred as Colorblock Jackets. In most cases the body of the jacket for all 32 teams are the same general color, with the opposing colors on the sleeves, collar, and hemline.
You can find jackets like this on http://www.nfljacketshop.com
Aeropostale because when theirs a sale on their clothes their t shirts price become around 5 to 10 dollars. Some shirts are plain tees and some shirts are tees with the aero postle title in front of it. You could also try hollister and forever 21 but hollister they hardly put anything on sale and forever 21 isn't very casual( you asked for a plain t-shirt)
I hope you found what your looking for:)