Welcome to the forum. You can buy the grates at bbq supply stores. Most guys making them seem to end up in the $100 to $200 range depending on how handy they are and how elaborate that want it to be.
If you make a vertical smoker you can put 1/2 inch rods through the barrel and use expanded metal cut in a circle just smaller then the barrel for the grill grates and do the same for the lower grate for the water pan....