I deal with at least a couple of suppliers who won't even sell to non-professionals. My hardwood supplier has a two hundred board foot minimum if I pick up and a five hundred board foot minimum delivered. I can buy for about one fourth to one third of what it costs at a lumber yard, because the yards buy from this guy.
My sheet goods supplier will only sell to the trade and sends out a salesman on the initial call to verify that you have working shop. The discount is not as big as it is in solid stock but I have access to products that simply are not available through yards. ie real cabinet grade plywood, sequence matched and numbered.
When I bought from Sherwin Williams, I ran about twenty percent lower than what Joe Homeowner could get the goods for.
My hardware supplier sells only to the trade and I can get solid brass knobs, 1 1/2" for less than a dollar apiece when I buy a hundred. I get Grass 180 degree hinges for about a dollar and a half each.
Being in business demands that you work your way upstream. You have to find out who the supplier is for your current supplier and see if that guy will do business with you directly.
It is amazing what paying cash upfront can do to open some of these doors.
Regards, Tom. Thomas J. Watson-Cabinetmaker Gulph Mills, Pennsylvania