Before I retired, my go-to method of finding esoteric parts was to contact the manufacturers’ trade association relating to the field that would use that part.
Those trade association exist to serve their members. If you write to them, using vague phrasing that makes you sound like a manufacturer, they will generally have a database that will help finding a source.
Google “furniture manufacturers trade associations”
I found this one (which may or may not be the right one to contact).
https://www.awfs.org/
I would write something like this:
Our company is designing a new adjustable height stool. We need to source hardware for this project.
Can you tell me when your next trade association trade show is scheduled and where it will take place?
We do need to get the prototype produced fairly soon. Do you have a database that would show a suitable vendor for these?
Also, is there a manufactures trade publication that we can receive?
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Note: The magazines (paper printed when I was working) are generally free. You have to answer a questionnaire to qualify. If you are uncertain how to answer, post the questions here and I will try to guide you. Your inquiry to the trade association should show an appropriate company name. Indicate that you are a start-up company so that they will not try to google you.
Something like “Farmhouse Seating, LLC” or “Seating, Inc.”
If you can find a physical example you might be able to find the manufacturer of the hardware on a label or imprinted on the stamped steel parts. That would simplify things too.
