To begin with I think you need to try another auto supply. You probably can't get any at the stores such as Auto Zone, you will need to go to an Auto Supply that caters to the independent garage trade. If that doesn't work, go to an automotive a/c supply.
Secondly, there are several sites you can go to for this. I would start at www.aircondition.com. The price varies, if I remember correctly, I found one for $14.99, but it might have been $19.99. You need to print the study material and read it. Then keep it handy and go back and take the test with the material at hand. It is an open book test. It's actually appears to be kind of a loophole thing, but it is totally legal.