Cliff (my bro) did the ones at Redan. Just a treated 4x4 and I think a bag of cement for each. The pipe is standard 1" pipe. Make sure you angle the top so the birds don't sit there and poop all over them. I'll ask how deep they are set and how much concrete he used. The last thing you want is to take all the time and material and have a kid hang from one and it topples. As far as prebuilt I don't know but the construction is very simple.
That was in, cocker block doesn't count!