man, if you can't find an old tire (I personally think semi tires would be awesome, but you probably couldn't plate load them without buiding some platform or whatever) you are doing something wrong.
I've got a 205lb tire sitting in my garage that I'm still trying to figure out what to do with it. Too light to flip it... save it for my boy when he is old enough to start? Drive a big mother f-ing bolt through it and turn it into the world's heaviest and most troublesome sled? Trying to drag it would probably be equal to a hummer pull... maybe worse because there is no way to build momentum. I've thought it might be a good way to train for pul events that require a harness and a rope to pull.
Anyway, getting your hands on a used tire should be far easier than getting a fat girl to eat cake