What product...

I'd buy as much Tanqueray No. 10 gin as I could and stash it if that was discontinued.
And English mustard, Smith's crisps (already endangered... I buy them when I see them. The only crisp - potato chip- which hasn't had a 'great new recipe' and still tastes as good as when I was a kid), Earl Grey and Lady Grey tea and razor blades.
 
Lindt chocolates. They are the best. Although there’s dupes for most things today and generic brands.