20' metal poll is mostlikely your best bet, I was lucky enough to have one leaning on the side of my house, I think from my brothers skating days.
Before I found that though I used PVC, I wasn't much of a builder and my rail did turn out like shit but I think the one of two times I actually made it on (yeah it was bad) it slid fairly decent. Or just hit up a construction site, those big white pipes slide easy.