This is how to get cheap and acceptable brake pads for your mountain bike disk brakes.
Step 1: Find the Braking Material
Probably you can buy some of various types of braking material in automotive stores.
But I just got an old used braking pad from automobile and sawed off some of material with angle grinder.
By the way, mind the thickness. Only 2 or 3 millimeters of braking material can fit into bicycle brakes.
Step 2: Glue It On
I took my almost entirely used braking pad and glued piece of braking material that fitted.
The best choice of glue would probably be some kind of epoxy, but I just used cyanoacrylate gel glue.
If you put some baking soda on a detail glued with cyanoacrylate, it should hold better. Then just clamp it. Every glue needs a good clamping.
Step 3: That's It!
In the end, round any edges with a file.
This s the photo of braking pad after a good amount of braking that I gave it in three rides.
As you can see, It only starts to get fit to rotor, so braking force is not too good it first, but then it gets better.