We bought a pile of these tubes from a fireworks factory who were not using them any more.
They come with end caps glued on one end of each tube. Unfortunately, we don't have a matching cap for them. But you can use a 2-1/8" disk, which we do stock.
Keep in mind that you will need to paste these shells in with tape, so whether or not you use disks or caps over the ends, everything will be hidden anyway. And we are certainly not going to tell anyone about your weird shell construction.