Thanks for your advice!
Unfortunately I'm from Europe, and I doesn't know any US collectors and / or marketplaces... (exception the eBay, but eBay is not very good imo).
Can you suggest me a specific website / marketplace for this articles?

I also have some letters from factories, as a "proof of originality" of the packages

kurszusz

Bro take these to the right collector and you may hit the jackpot. Especially in the southern United States like Kentucky and Arkansas..