“Buying a gun” is a meaningless gesture. It’s like buying a stove for a person who does not know how to boil water. Since your husband already owns one, and you have access to it, learn to fight with it.
Many will suggest courses that take several days at places like Front Site, Thunder Ranch, etc. These are excellent if you have the time and the money, but if you don’t, find some local teachers that will give you a few hours and teach you how to use deadly force.
Not to mention too, if you do something stupid with your gun like hold it right up to the perp’s face and he’s martially trained, you’ll be without a gun and you won’t have MA training. Train in both for maximum efficiency. Guns aren’t magic wands.