When it comes down to it, honestly any large enough pot with a tight fitting lid and a canning rack can suffice as a Waterbath Canner, but it is sometimes hard to find pots that are big enough. This is why there are special pots called waterbath canners made for the boiling water canning method.
There are a few different types of waterbath canners. You have Granite Ware Waterbath Canners which are usually available in the following sizes: 11.5 quart, 21 quart, 33 quart. I have also been seeing stainless steel waterbath canners with clear glass lids, I have seen these in the 21 quart size.
Also, many people who pressure can as well will use their pressure canner for waterbath canning methods. The way to do this is to be sure that your lid on your pressure canner is NOT sealed when using it for a waterbath method. Your lid should only be sealed if you are pressure canning.