For example if a leaf from a certain tree works better for a toilet paper substitute, then let me plant it today rather the having to use a rock.
For that specific question, I have picked up some answers in odd places.
One was in book about using bathrooms in other countries. Some places use water (splash it on the correct area, shake dry.) The book was called "Going Abroad: The Bathroom Survival Guide," by Eva Newman. (I found it fascinating to read.)
In some other book (whose title I have forgotten), I have read of people using corncobs or small sticks.
Like another poster said, I have also read of using newspapers or pages of catalogs or of books.
When anything made of cloth is completely worn out, you might cut it into pieces and use them for this purpose. (But consider whether there is a better use, before doing that. Sometimes a worn out garment can become a smaller garment, or a tote bag, or pieces can be used to patch a different garment, etc.)
As regards using leaves, you could look for hiking and camping resources. It would almost certainly depend on what plants are local to you. Do be sure to check for spines on the leaves (top AND bottom), and make sure they are a kind that does not cause irritation when rubbed on the skin.
I think it would be ideal if you can use leaves of a local weed (so you want to pull it up anyway), or from a plant you would already be growing for other purposes (maybe the outer leaves of a cabbage, that are often thrown in the compost pile instead of eaten?)
I could see corn husks being useful, especially if they are dried, in areas that grow corn. I am sure people using corncobs was partly because they already had them in quantity from growing corn for people to eat.
This may be obvious, but none of those alternate things should be flushed down a typical toilet. So plan on something (compost bucket?) to collect them, even if you still have water to flush the toilet.
You could also consider something reusable and washable, similar to how people use handerchiefs (vs. kleenexes) and washcloths (vs. paper towels.) A few years ago there was a toilet paper shortage in my area, and I gave this idea some very serious consideration! (I didn't actually have to try it because the stores did get more toilet paper before I ran out.)
Im looking for a forum or some literature on how people lived and survived in the old days.
Stuff like what did they use for toilet paper, what did they feed all their animals?
There are some things in "The Encyclopedia of Country Living" by Carla Emery (although I also found some things that are definitely modern, and a few things that were just plain wrong.)
For feeding animals, do some research on the individual animals. For example with cattle and sheep, some people in recent times have been raising them 100% on pasture (no grain, and in some climates no hay either). If you have the right pasture that can be an option to consider.
Try looking for "permaculture" and "homesteading" forums or books. Some of them will be excited people with no knowledge, but some can have helpful information.
I agree with the previous poster who suggested the "Little House" books by Laura Ingalls Wilder. I especially recommend "Farmer Boy" of that series.
Books on saving money may have ideas you can use too, like re-using something (maybe in a different way) rather than buying a new one. For example, the "Tightwad Gazette" books by Amy Dacyczyn have suggestions for washing and re-using plastic bags. That exact idea would let you have the benefit of plastic bags for holding or storing things even if you cannot buy more at the store (of course the bag wears out eventually, but at least you have more time to think of alternatives.)
Double-checking information and testing things before you need them will help no matter what your source. Expect to wade through lots of information to find the bits that are useful to you. Pay special attention to climate or weather details (hot or cold, snowy or wet, can make enormous differences.)
