It took me personaly months to track back to a specialist manufacturer.
It might be easyer to explain how we get them made with a picture.
You need to primarly find a heater core/radiator manufacturer that will 1:talk to you 2:help with final design.
You will need to submit a rough drawing with dimensions, as you can see in the picture, each unit is made up from various parts.
The cheapest option is to ask them to source std parts( core fins/end tanks/channel etc) that are as close to your specs as possible.
From there you do very little - they make up a development unit , and send it to you for approval.
It takes along time (well for me it did) to find a Co. willing to & culd do what I wanted.