Where can I find very strong wall brackets capable of supporting a punch bag?
I tried to think of an adaptive use of some common building product but drew blanks. That may be why purpose made ones are expensive. If it really seems exorbitant, you might be able to get a local metal shop to fabricate one for less. For that matter, someone with decent carpentry skills could build one from wood. The joint designs would be critical to ensure they are strong enough and don't vibrate apart. Something of welded steel would be a lot more idiot proof.

Is there a possibility of hanging the bag from the ceiling? This is more likely a DIY solution. There's been a few questions recently about this here. The key is to use multiple fasteners such as lag screws to engage multiple ceiling joists to spread out the load.