This was such a tough product to find.  I took a flyer on this company when I built my wash bay in early 2016.  These are custom-made to our spec.  Keep in mind that these are insanely high quality stainless steel, hence the cost.  These will last longer than you.

I've listed the most common sizes, but I can have them made in pretty much anything you want including slightly less expensive T430 or marine grade T316L stainless steel.  Floating designs are also available (at about 2 times the cost). Email support@obsessedgarage.com if you need something different.

The size I have in my garage is 8" deep x 32" wide.  I was able to mount the brackets directly to a stud to ensure they never move.  8" is the perfect depth for gallons from Adam's, Griot's, Chemical Guys, etc.  If you are a big CARPRO fan, you may want to go with a 10" depth.  CarPro gallons fit on the 8", but there is about a 1/2" overhang.

If you're looking for the shelf that I use for mounting a pressure washer, click here. It has the same construction as these, but is specifically designed to fit a pressure washer.