I'm not too sure about now, but the last I knew the Cobett's were only available from the inventor/manufacturer. 

I went to his farm to pick up 2 large and 2 small ones a few years back and I want to say the total for all 4 was $2500 ($725 for the large and $525 for the small [I think]).  They're not cheap, but when the weather gets cold around here, they are the only waterers I don't have any worries about.  Peace of mind for livestock water is worth quite a bit when there's plenty else to do and it's cold as he!!.

I'm thinking the last time I seen one of the brochures the small one was around $550 give or take, but maybe higher by now, it's been a while since I've checked.