About 40ft of 3/4" manila rope. Simply walk it around the base of a tree, flick the rope up a few times to get it over head-height, tie it off with a timber hitch, and repeat on another tree 12-18ft away. Like I said, it's much less work than a slackline.
Also, be prepared for some sore feet at first. You might want to try wearing a soft-soled shoe unless you already have very hard feet.
And yes, I'm actually holding that handstand. Get on my level.