I am not sure if it is needed. I heard stories about it but was never asked to show it, nor was there anywahere I could buy it, and I went through the same way you are going.
I crossed the border between Chile and Arg quite a few times on the way to the southernmost tip and never once was it available to buy, and I do recall asking at the border about it and was told no problemo.
So I went without, but do a search on this forum and you will find a few threads on this subject and I have a small inkling that I saw company names mentioned as well.
Cheers
TS
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