It is all free. Get tires at tractor supply / construction equipment companies. Used are free.
Fill a bag with sand, go find some big rocks. get a rope and harness and start pulling stuff.
Rocks you will find wherever large rocks are found.
You can get a bag from anywhere. I like the Green bags from the army surplus stores.
Sand is cheap by the bag, or free from property that has sand on it.
Rope you can get at the hardware store.
If you are looking for farmers walks or yoke implements, you will have to get more creative of have them built be a welder. (you can get some from some companies, but the price will be high) The same is true if you want a log.
Also remember that strongman training is not a replacement for gym training.