Rubber, also called India rubber, latex, Amazonian rubber, caucho, or caoutchouc,[1] as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand and Indonesia are two of the leading rubber producers. Types of polyisoprene that are used as natural rubbers are classified as elastomers.

Currently, rubber is harvested mainly in the form of the latex from the rubber tree (Hevea brasiliensis) or others. The latex is a sticky, milky and white colloid drawn off by making incisions in the bark and collecting the fluid in vessels in a process called "tapping". The latex then is refined into rubber that is ready for commercial processing. In major areas, latex is allowed to coagulate in the collection cup. The coagulated lumps are collected and processed into dry forms for sale. (From Wikipedia)

Nice to know..... But what good is this knowledge if you have a Fiat 850 with worn rubbers? Nothing of course.

What is useful is a company that can supply these parts, made on the original machines that Fiat's suppliers used to work with: Cicognani.

I got to know them years ago at the fair in Padua. A huge choice of rubber parts for just about all Italian brands and types. I have ordered for my own cars from them several times (the Fiat 850 and my Fiat 124 Coupé) and every time it fits like a glove.

I have the price lists of January 2022 available here:

850 berlina cicognani850 sportcoupe cicognani850 sportspider cicognani

They aren't cheap (their price lists include VAT for most European countries and postage for everyone) but the assurance that the parts will fit is well worth the money.