It is not a small amount of people it did not work on, The tests done on rats and other animals failed to work on humans over 80% of the time. That means over 80% of research is wasted and the whole process starts again. Because of this, A large amount of money is also wasted because the research was completely useless.
As for finding people to test on. Death row inmates can choose either the death penalty, Or human experimentation. They will be given a short explanation as to how the experiment can benefit society. Either way, They will die. However this system allows for the science industry to move forward significantly as they are able to test products a lot more accurately.
When you stated that the reason they do not test on humans is because the human may not have the disease, The same goes for rats. The scientist gives the rat the disease, Then proceeds to perform experiments on that rat. For humans, The scientist would give the death row inmate the disease in order to start the experiment. Neither the rat or the human needs to already have the disease in order to begin an experiment.
This new procedure may cost more, But the benefit of more accurate results would benefit society dramatically.