Often the actual manufacturer is not setup or able to deal directly, so a middleman/distributor is needed.
Alibaba is the best place to start as mentioned above, but is not a simple shopping cart site but rather a way to negotiate and setup a trade agreement with escrow facility. I have used it a few times with no problems, but often I find it is just easier to find a good seller on AliExpress and after a trial order or two chat with them about bulk pricing. Other times, I've managed to find the original manufacturer based on the markings/labels on the device, tracked down their website and contact details, and then negotiated directly typically using PayPal for a little extra security.