The use statistics are: 35 for indispensible vs. 1887 for indispensable, so it is overwhelmingly in favour of the latter. Google ngram confirms that this is not a particularly recent trend, and that the two words were used competitively until 1840, where usage seems to have settled on indispensable.
Oh, sorry i wrongly typed "indispensable" to "indispensible". The original sentence was " His article was indispensable to the company". But I came to wonder if it was possible to change 'to' to 'for' without changing the meaning, or if the meaning changes, what would be the difference. This is kind of a same question with the second question ...
the monkey wrench being the most indispensable item for a plumber scissors being the most indispensable item for a tailor etc. A different but equally useful direction of meaning would be a term for an item which is guaranteed to be present in a certain line of business or endeavor. For example:
Indispensable nutrients (including amino acids) are those without which life is impossible. Indispensable nutrients that cannot be internally produced, but must be consumed from some external source, are called essential. " Essential " is a scientific term of art in nutrition (i.e. it has a fixed, universally understood meaning); " indispensable " is not. As a result, the two are sometimes ...
It is indeed "not indispensable": Most of the luxuries and many of the so-called comforts of life are not only not indispensable, but positive hindrances to the elevation of mankind. (Google Books) Thoreau was a strong believer in simplicity of living, so you are correct when you say that "not only indispensable" makes no sense.