Making things in the US is tough. And much more expensive than overseas. But doable, if you design for manufacture and plan the pricing for the cost.
For China, Alibaba is an OK place to start looking. Pay attention to Factory listings vs Trading Partner listings. If you are a designer, you may want direct access to the factory.
Finding an agent may be a good place to start out - check out http://www.e-bi.net. I’ve talked to them, but have not worked with them, so go at your own risk.
Go to a tradeshow in China - you’ll learn a ton and meet all the factories you can imagine.
Trust and Communication are important in exploring any new business relationship, so if you can find any recommendations from other designers for factories that may go a long way. What materials/process do you need?
It’s a huge topic, so if you have a smaller specific question, I may be able to help more…