Hi @sagaxai,
You'll likely want to look into becoming an Instructure Partner. Depending on the partner level you select, you can get access to a Canvas sandbox instance to do all of your app testing. Please note that this is not a free program, so you'll have to decide if it's worth it to you to invest in.
The only other option at his point would be to try to find a school that would partner with you and give you some access to their Canvas instance, but I think you may have a very tough time doing that because of the level of access you'd need and the data that would be exposed to you by granting that access. Speaking as an institution Canvas Admin myself, I can tell you that my institution does not allow this kind of access to any external partners, as it's just too risky.
Hopefully this info helps!
-Chris