Similarly, there are probably scores of mattress manufacturers out there, most are regional. They are mostly “cut and sew” operations, and they buy the spring innards from one of the handful of spring frame manufacturers.
The biggest of them was a customer of mine when I still was in the business. They were so big that they owned the wire mill that produced the spring wire.
I think a similar situation happens in the furniture business. They have a twice a year trade show for that industry. I believe it is held in North Carolina. If you can talk your way into the show, you can learn a lot.
Google: upholstered furniture manufacturers trade association. They can tell you which are the trade publications and when and where the next trade show is.
Just tell them that your uncle was in that business and now wants you to join him. And that you are trying to learn more about it. At the very least you will learn of some wholesale suppliers.