There are a few places who will sell flag shirts.However, most of them tend to have the year written on them (since they only make them around Memorial Day, 4th of July and Labor Day). They are not make year-round, so if you're looking for a flag shirt outside of those holidays you might have a hard time finding one. Instead, try making one. It's cheaper than purchasing a flag shirt, and you can make your shirt into anything you'd like. It is a fun project for kids to participate in as well!
Puff Paint Method
Put the cardboard inside of the gray T-shirt. This will not only help the shirt stay in place, but it will also prevent the paint from bleeding onto the other side.
Map out how big you want the flag to be.The flag will be a different size for the different sizes of the T-shirt. Typically, leave at least two inches of fabric from the side seams on the front front of the shirt unpainted.
Start with the blue paint, and draw a large square/rectangle in the upper left corner of the t-shirt (which will be the right side of the shirt when worn). Color the whole area in with the blue paint, and allow it to dry completely.
Draw on 50 little white stars inside of the blue square/rectangle.
Start with the red paint, and paint two red lines from the blue square out towards the right side of the shirt (using a ruler may help). The two lines should be inch apart. Color in the lines with the red paint. Underneath the red stripe, repeat this with white paint.
Continue this until there are seven total red stripes and six total white stripes. When finished, allow the shirt to dry completely before wearing.Wait a whole day for it to dry.
Sewn on Method
Cut the fabric pieces: cut a blue rectangle approximately six inches long and four inches high; cut four pieces of red fabric into six inches long by one inch high; cut three pieces of white fabric also into six inches in length and one inch in height. Then cut three more pieces of red fabric into 12 inches of long and one inch high; and cut three more pieces of white fabric at the same proportions.
Cut 50 little stars from the remaining white fabric (using a stencil may help to do this). Stitch the white stars onto the blue rectangle using the white thread.
Stitch the smaller strip of fabric together. Start with the red strip and sew the white fabric to it using the white thread. Then sew another red strip to the white using the red thread. do this until all the small strips are sewn together. Repeat this with the larger strips of fabric, starting with the white fabric.
Sew the smaller strips under (only a centimeter) the right side of the blue rectangle using the blue thread. Then sew the larger strips under the rectangle with the blue thread and under the last strip of fabric from the smaller strips (it will be a red strip, so sew it on with red thread). The flag is now complete.
Sew the whole flag onto the t-shirt. The shirt will then be completed.