You could just collect some of the extra wood (2"x4"s, 4"x4"s, pallets, plywood, other useful-looking junk) from construction sites and nail it together to make skinnies, inclines, steps, ramps, or whatever else you think would make a decent trials obstacle.
You can go behind most plazas/malls/stores and find a bunch of pallets - the only problem is, sometimes the stores have to pay a deposit for the pallets, so you should probably ask them first.
As well as all the ones already suggested, I suggest you have a go at designing some yourself…it’s half the fun! Hopefully www.unicycl.2ya.com will be up and running again soon.
i have a bunch of pallets, some 2x4s, a coupel spools and a junk car laying around in what i think is a semi-difficult configuration. nothing’s nailed together and everything can be rearranged in a matter of minutes. just get as many materials as you can, and put stuff together, don’t worry too much about measurements and what not.
I bought some 8’ lawn timbers from Home Depot and a couple of 2x4s. The lawn timbers are about $3.50 each. I cut the 2x4 into 18" strips to use for braces - one on each end. (I already had some deck screws.) The braces raise the lawn timbers off the ground a couple of inches, and provide some much needed stability. I can move them around the driveway and yard in any configuration. I have eight 8’ lengths, so it’s pretty flexible.