I found them on the GMCs more common than on the Trailblazers at the local pic a part. They were all of $8 from the pic a part yard by me. I did buy new bolts for mine because I found the factory ones off the Envoy were rusted badly on one side.
The one that simple-engineering is selling is made of parallel square tube instead of channel and therefore is considerably stiffer than the units made for the Trailblazer and Envoy. The GM parts must be for the Trailblazer EXT or Envoy EXT since the other models only came with one mounting hole and got the same bar we got.
My opinion on it is that it helps reduce body noise and helps stiffen up the truck. When I back out of my driveway there is a bump between my drive and the road of about 2-3 inches. After I put the brace on it makes less creaking noise when I back out. That alone for me was worth the time and effort to install the 4 bolt cross member. You would think that with all the cross members these things have it would not help but it does. I have looked at the pictures of the frames and there are I think 8 cross members on the frame. That should be enough to keep the truck as rigid as could be. I was skeptical and went to a local u pull it junk yard and got the cross member off a GMC Envoy. It only cost me $8 and I bought new bolts at the hardware store which cost me about another $8. So I figured for the cheap cost I would try it. I was surprised to see it made a difference