I saw frames for sale at Interbike (just browsing and didn't even ask which motor fit), but that's it. Your best bet might be to purchase a bike and sell the unwanted parts. Companys like Frey Luna Bike, ... might sell a frame-motor-battery, but it won't be Brose, Bosch, Shimano etc.
I don't know what you are thinking as far as motor/battery/controller, but the MSRP on that stuff is ridiculous for the "Big Four" (Shimano, Bosch, Yamaha, Brose). For example, the 500WH battery for Bosch is $900, the motor is $1100, the controller and associated stuff will set you back around $500.
For a mid-drive, the frame has to be built to match the motor. Kinesis lists them in their catalog, but they don't sell to individuals, only OEMs.
I think Dengfu is an OEM as well, which means they would have a minimum quantity you would have to order.
There was a guy at Veloswap in October selling carbon 29" hardtails he had imported from China. He had miscalculated the demand and was stuck with a bunch. I think he said he bought 100 of them. He was selling frames for $250 each by the end of the day, full bikes for $750. They looked fine, but who knows?