First before answering the posters questions there are some relevant questions:
What type of boat
How big (length) is it.
Is it gas or diesel
Where is the boat
located? (the country, you don't need to be any more specific than that)
Is it a recreational boat, or does it carry passengers for hire (this includes 6 or less, commonly called 6 pack, which are usually recreational boats)
All of these determine what rules apply. A recreational boat has vastly different requirements than a passenger vessel. A small recreational boat 16 to 26 ft has different requirements than a 45 or 50 foot boat.
Plus that, as someone said insurance
companies can require what ever they want. It doesn't make it a reasonable request. Try to get some clarification from the insurance
company or agent.
Now for you remark about buying
from West Marine
or other Marine
supply, yes that is what most people do. They buy the extinguishers they need and install them DIY