Ebay. Half inch thick carbon plates are apparently common in glass and solar panel manufacturing. Get them as thick as you possibly can, 1 in is not unreasonable. Also get the biggest plate that you can. Mine has six bolted the whole way around the barrel. Electrolysis works by line of sight, and the plates do degrade over time, so you want as much starter material as you can possibly get in there to avoid having to change plates too often.
You get some sludge that settles out on the bottom, but the water just stays murky black. I have a bung (lol) on it that let's me hook up a garden hose and drain it. Tbh I never change the water unless I have to work on a plate, then I just drain, scoop gunk, refill, and add sodium carbonate. You can get sodium carbonate in bulk from ebay/Amazon. I bought a 5 gallon bucket a while ago. Still have a lot of it.
