You can find suitable flag cases for sale here

a size generally used in military applications

This 8"x12" US flag features printed stars and stripes and is finished with a strong polyester canvas header

This historical 3'x5' 13-Star flag is made from 200 denier nylon fabric and features thirteen appliqued stars and thirteen sewn stripes

Finished with a roped reinforced heavy-duty polyester header on the hoist and several rows of commercial lock-stitching and strengthening corner-patches on the fly end

20' Fiberglass 3-piece Flagpole nautical-sized-us-flags You can find suitable flag20' White Gel Coat Fiberglass Villager III Alliance 3 piece Residential Flagpole; 2" Top Diameter x 3. 5" Butt Diameter with 3" gold anodized ball, single halyard fixed truck assembly, polypropylene halyard line, (2) plastic snaps, collar, 6" cleat, and PVC ground sleeve Manufacturer specified load capacity: 130 mph unflagged & 15 square foot of sail @ 100 mph (3'x5')