If you will scroll down a small ways you will find another person hunting for the same thing. They have been all over the place and to no avail.
Since you built this home yourselves, you have to be an experienced carpenter. I would feel that with one by fours, you could prefab a beam yourself. It would be hollow but made from 1x4's and some trim molding. Should be very easy to fabricate.
I would make the beam downstairs and when all three made, stained, sealed, and all, just hang them up. Apparently the bad rock job was done at the seams and the seams are over the joists or studs. Just screw them in and walla. your fixed.
These foam fake rough hewn beams were quite the thing about 10 to 15 years ago and they were found in home centers, but since, they have just vanished from the face of the earth. If you do find them, be sure and come back to us with an address or two.