I fought with bad options for this for many years. the best solution I found was this crutch bag. But it's expensive for what it is and did not attach well to my low profile manual chair. The bottom of the bag dragged on the ground a bit. But if you have a taller chair with push handles it may be a solution for you.
I finally bit the bullet and bought a pair of folding crutches from Walkeasy. They fit in a backpack pretty easily.