I think these kinds of exercises are based in availability and money conservation. If you live near or on a farm where they have an old tire you can use, use it. But any similarly awkward sized, heavy thing will work. I'm big on trying not to spend for something you can have for free. Use a heavy bag as I mentioned, tie 2 together to make it heavier, etc. Use what you have. I like the exercise but I'm not going to spend that kind of cash on tractor tire that's not even going on a tractor. ( if you get my point )
_________________________
There are no PERFECT techniques, only perfect execution for the situation at hand. ~Corwin