@lowensby,
It is ridiculous for companies to ask employees to take drug tests. It is bullshit. It is none of their business, and studies show it doesn't work to improve productivity.
I wish you didn't care about this job and could find a job with a company that doesn't test. I am fortunate to work in an industry where companies who drug test won't get any employees. If a company says "drug test" I walk away. This works because most of us will walk away (if I am hungry enough, of course I do whatever they ask).
I did a quick search. It looks like California NORML (a pro-marijuana group) has a paper on drug testing that might help. It costs $10 and based only on their reputation I might take this one seriously.
I wouldn't trust the products that my quick search brought up. It might be the case that you will just have to take the test an pray for the best.
There are an awful lot of people who use marijuana casually. Employers who don't accept that are going to have trouble finding good workers.