You must not have looked very hard, becasue I typed in "building snowboard rails how to" and several things popped up including a you tube tutorial on building a rainbow rail using an old trampoline frame. I can vouch for using a trampoline myself as I did this recently to build a kicker and drop in ramp.
First, find someone that has an old trampoline they want to get rid of- make a deal with them that you will make it disappear for them for free, or check craigslist for a used one for cheap$$$. As a last resort, check the trash cans in neighborhoods on trash day after a big wind storm to find bits of damaged discarded ones.
Then, use it for the galvanized steel tubing. Lots of long straight pieces for rain bow rails and curved pieces to make nice kickers. I did this with ours after it got damaged beyond repair from the wind and was really happy with the results. All you need is patience, a drill, hacksaw, nuts and bolts, 2 X 4s and some basic skills. Other than that the only thing holding you back will be yourself.
Second, Snowboarder Magazine had an article on building a box in their Droppin Knowledge section. Hop on their website and other sites to use the search function for building ramps, backyard set ups, rails, boxes, etc...you get the picture. Use your resources like the web and be creative...garage sales are full of used stuff for cheap that can be used.
If I ever get motivated enough, I'll post my backyard drop in and kicker on here. Good luck.