well home depot, lowe's, or any kind of home improvement store will be a good place to start. materials wise it comes down to how much you want to spend. ply wood and 2x4's are a good place to start with. then you can get creative with types of tubing or any material that will slide. i haven't had any issues with synthetic/composite decking material. but, i am still messing around with things. idk my back yard is pretty fun and always growing.

Sweet, thanks for the help. Does home depot or lowes have that HDPE plastic or something similar to use for the top of the box?

HDPE is usually not available at those big box stores. A fast google will probably find someone in your area that carries it but it is pretty expensive. Timh810 mentioned the composite decking material and its a pretty good cheap alternative. Weve used it for a few boxes both for skating and boarding. It holds up really well and is pretty "slick" too. The one thing you will have to do with the decking stuff is make sure you buy the kind that has a smooth underside. Most makers stamp a fake wood grain into theirs so you want to flip it over and use the smooth underside for the top of your box/rail.

also hdpe can get expensive.

Sweet, thanks for the help!