I'm in the paint biz, I'll give you the skinny on who makes what.
SW does private label their paint under several brands (MAB, Brod Dugan, Duron, Mautz, etc..) except for:
PPG
ICI (Glidden)
Kelly Moore
Kwal
Ben Moore
Farrell Calhoun
Anchor Paint
HIS Paint
Mobile Paint
Valspar
There are a couple of guys on the left and right coasts that I can't think of right now but suffice to say, SW does not make 97% of the paint in the country.
The retail chains like SW and Kelly Moore charge no more for their paint than the big boxes, they only sell cheaper to the contractor to keep that business in their store (and out of the competition). None of them make much money selling paint to contractors (less than 5% net profit in many cases). If you want a better price, ask them for a contractor price. They can only say no, and you just might save 20%.
Stick with the paint stores, not the big boxes. If you need advice or a special color, Lowes or Depot will look at you like you've a horn growing from your forehead. They don't like to sell anything but straight white and colors that the computer can mix for them.
The local stores go out of their way to make a special color paint and will make you a custom color stain to boot. They will tell you exactly how to apply and how to fix a paint problem. Try that at a big box.