I strongly recommend to fly over and know those guys personally.
I lived there > 5 years and know what I am talking about.
See the "factory" and do not be surprised not to see what you are
expecting...most are working well anyway.
Getting fast response is not what you want. you want a good product and
if you are a nice guys, you will not want children building it, neither
them dropping their chemical on the lawn.....
The only way is getting there....Everyone can send nice mails in short
time and even lye you with nice references and certificates.
There are literally millions of this kind of factories.
Go and get in contact in Shanghai, Beijing Dong Road 666 and there you
will find dozens in just one building.
They even do reverse engineering of any product you bring in, in just a
few days and offer you the complete service (board, components,
assembly, housing, shipping.....)
WIth a lot of experience in Chinese outsourcing.