Look online. Or make a metal brake and fold your own sluice for $20 of 3mm aluminium. Hell, I've seen mates backbend zincalume fencing panels to 'flatten' them then turn that into a sluice box, or even just use the in-situ bends to make long toms out of them. No $10-20 sheet of aluminium is worth the big price tags being asked. Unless you want to spend thousands to make cents I strongly suggest making your own, well, everything.
Or at least look around online, those 'Gold Crocs' were made under various names, but all came from the same factory in China, just different resellers bought them ten at a time, stuck their own stencils on them, called them something different and sold them for $300 saying they were something new, a fluid bed sluice. In reality they paid $10-20 a unit and even the 49ers used them and called them boiler boxes (albeit the piping is 90 degrees to the ones the old timers used but the principle of the physics behind it is identical).