If you find source from china; alibaba, makeinchina, globalsource make your first choice before.

But now there are a new model b2b platform for your choice--buysino.com. Which claim to be a free b2b platform, and it 's distinguish from the traditional b2b platform, it not only show the sellers and buyers demands, but also aid the buyer and seller finish the trade, you can finish all of the procedure on the site one stop. you needn't fly to China, and check out the manufacturer's factory. You just negotiate with the buysino, the buysino will guarantee you receive the goods, if you still can't sure the product quality, you just need to see the sample at the exhibit room in US.....

I believe this model of b2b platform will have a good future, and I know the new version of the site is testing, you may experience the new platform at May, but you still can visit the old version.
The Leading Free B2B Marketplace