mtngun
Addicted to ArboristSite
It's used mostly by industrial/commercial electricians, so look for an electrical supply outfit that caters to contractors.Not much luck on long Unistruts at the lumber yards here. Not sure who else would carry them.
Also check out your local steel yard, they sometimes stock it.
Home Depots in this area carry 10' unistrut, but not 20'.
You might be surprised how much it costs.
Your sandy, rocky soil should drain well.
I've got hardpan clay here. Thought about coating my timbers with roofing tar and burying them like a conventional pole barn, but it seemed too risky considering the terrible drainage.