It is easy to make your own piping bag - just use a plastic bag or baking paper. I recommend using a plastic bag as you can secure the sides easily with tape. Some baking/wax paper might be tricky as tape will not hold and you have to use staples. The illustrated steps below are doing using a sheet of baking paper as it is clearer in pictures. You don't need to be an origami master - 5 steps are all you need!
1. Start with a rectangular piece of paper. If using a plastic bag, cut it and lay it out flat.
2. Bring the bottom right edge of the paper towards the top left corner. The idea is to create a triangle as seen below
3. Fold in the remaining flap on the left. Secure with tape.
4. Flip the whole bag and you will notice a small triangle flap at the corner. Fold that in and secure with tape.
5. Using a sharp scissors, cut of the tip. It should be big enough to fit your piping tip.