Terminology is very specific to geography, but if you were in my neck of the woods you are asking for two different things at the same time. Slab wood here is the wood that has bark on one side and generally isn't any specific width or thickness as it is what is removed to "square" the log for cutting a specific dimension of lumber.
Rough cut wood is the specific pieces of lumber that is cut from the "squared" log.
I use latex paint, whatever color I have on hand, white seems to absorb less so I don't have to remove as much when squaring the ends later.
To answer more of your question, yes some small mills sell rough cut wood, some small mills don't sell rough cut wood. The reason for that type of answer, some small mill guys like to cut lumber so go to the site and mill wood for the log owner. Other mill owners like to buy the logs and sell the lumber or they cut on a percentage and sell/use what they earn. It is a varied as the number of mill owners out there.
In many mills, some will slab a log til they get a square cant, the term for the log after it is squared. I may make 3 passes til I get what I want, and these "slabs" get cut for firewood. I would be surprised if you don't find a mill in your area that will have these. On the wood web, in the sawing and drying sub forum, you will find a way to locate sawmills in your area. Good luck.
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