“The “standard” substitute for Wall Ball is either dumbbell or barbell thrusters. Since you can’t (or shouldn’t, anyway) actually throw the dumbells in the air, use about twice the specified ball weight (40 lbs or so instead of 20) and do them as explosively as possible . . . but it’s still quite different.
Better to follow David Heyer’s directions for a homemade medicine ball:

1. take an old, or cheap, basketball
2. cut a slit in it
3. stuff with sand
4. sew or glue the slit closed (optional), and then tape up with heavy tape (Americans call it duct tape)

This gives you a perfectly functional 18-22 pound ball (9 or 10 kg) for under $4. ”

Hope this helps a bit although we did do a few db thrusters yesterday.