start by pricing material for it on a site like
speedymetals.com, most suppliers will want to sell you full lengths of material rather than the sizes you need. a local supplier is
sullivanmetals.com, they also do scrap, you might be able to find some of the pieces at a scrap yard if you're really trying to cut corners. Fair amount of mitering and welding involved, once it's all cut and machined, the welding should be fairly straight forward but will take a bit of time. In a fully setup shop could be made in a few days if all stock is cut to size and only cuts lefts are the funky angle miters.
how do the wheels attach? any pictures of them finished and in use?