Have door, need to build roughout and frame
#1

I am building a 4th bedroom by building new interior walls. One wall will have a 36" door, solid core (heavy!) in it. I think I can figure out how to build the opening from the books I have.
When I look at prehung doors I see the casing; it appears to be 5 or 6 inches wide x 3/4" thick with a 1x2 size board around the middle to serve as the doorstop.
Where do I buy such lumber. Home Depot? Local Lumber Store? Or, is there a premade frame that does not already have the prehung door?
When I look at prehung doors I see the casing; it appears to be 5 or 6 inches wide x 3/4" thick with a 1x2 size board around the middle to serve as the doorstop.
Where do I buy such lumber. Home Depot? Local Lumber Store? Or, is there a premade frame that does not already have the prehung door?