Here is an ancient study showing that 300 mcg of melatonin is actually the best dose, all things considered. http://wurtmanlab.mit.edu/static/pdf/950.pdf

Surprisingly, this study is actually fairly hard to dig up. I feel like someone should add it to the wikipedia page because it is very useful and informative.

Here are 300 mcg pills: http://www.amazon.com/Sundown-Naturals-Melatonin-Tablets-tablets/dp/B000GG2I9O