I've used a lot of stuff from my place. I was very very pleased with the results of using black locust (great for fence posts) but they rotted out within 12 years because of so much ground water. In the future? I will use concrete as piers to put the wood uprights on.
Alternately, I've used tv antennas rehabbed in PIECES like 10-15' tall for the uprights and THEN concreted in... Last longer and the triangular things are very durable and the load seems to be good because of the shape. Hey, if it will support an antenna however much into the sky? It'll be a great support for my top-bars. I used top-bars of long, thin black locust and arranged them as a sunburst. Really neat, broke the hot sun in the summer (once the wisteria grew) and then let in the sun in the winter.
Sadly I've not completed my re-do of the sun-shade yet but am using front uprights of some old gate type things that are 8' tall. If I can ever figure out how to dig the footers and support their weight properly, it will be wonderful again. Don't forget that anything you plant to grow UP on them will be added weight as well.
Have you looked at Garden Junk? http://gardenjunk.org/photos/default.aspx
or this one? http://ths.gardenweb.com/forums/
check out trash to treasure forum...
and one of my faves?
Anyway, I plan to make a trip to the local junk-car & assorted junk metal stuff place to see what I can find to make into a second gazebo. I bought a commercial one some time ago, but it's so pedestrian. I've never assembled it! LOL. It doesn't go with my early American re-hab look.... Have fun!