I think you have to be employed in that particular airline, as I remember when I was a mechanic, and an open for bidding for inspection personnel came, you have to have a number of years residency + the necessary experience, of course the educational attainment comes first. Of course you must possessed A&P license. If you're qualified they will give you the trainings you needed.
First get a A&P, then get some experience, then get the IA.
For me I want no part of a IA, I'm happy with a A&P.
it depends on the size of the company, plus you have to be an A&P mechanic first.