I happened to be at an apartment complex that was rebuilding their decks. They had just finished and the dumpster had the ends of the new 5/4 decking they used. I snagged about 80 - 2 foot long pieces. It should make about 40 feet of nice boardwalk.
In the past, I have been able to get some old deck material that was still in good shape and have made numerous projects with it.
So, keep your eyes open for construction in your area. Most people would be happy for you to save them on disposal charges. Even better, contact a local construction company and let them know you could use some "scrap" lumber.