Do you have the financial resources to cover the costs of a criminal or civil trial following a justifiable shooting? It may be better to be tried by twelve than carried by six, but if that trial leaves you financially ruined for life then that isn't much of a victory. Even a clear-cut justifiable homicide will require a lawyer working on your behalf. Here's a little known fact: remember that part of the Miranda warning that states "if you can not afford a lawyer, one will be appointed for you"? Just because the court appoints a public defender, that doesn't mean they're free. You may pay on a sliding scale based on your ability to pay, but you will still pay for that attorney's time.
The logical first step is to look at your homeowner's or renter's insurance for coverage. Most don't provide enough coverage. Expect to need at least one million in coverage to defend against that wrongful death lawsuit the criminal's family will file. Depending on your economic status, two million or more may be required.
The NRA does offer self-defense insurance but the coverage amount is way too small to provide the coverage you need. That's where umbrella liability policies come in. Umbrella liability coverage starts where homeowner/renter insurance stops. It's relatively inexpensive but there are some critial "gotcha's" you have to be aware of.
Usually umbrella coverage protects you against unintentional acts. Self-defense by definition is an intentional act and therefore isn't covered. There are two ways around this.
The first (and perhaps best) is if your insurance policy states in writing that acts of self-defense are excluded from the intentional acts exclusion. I know it's confusing legalese, but this negative of a negative means the policy will cover you in acts of justifiable self-defense. Don't go by what an insurance agent says. If it isn't in writing it doesn't exist.
The second means of coverage is by state statutory or case law. Some states have laws that specifically state self-defense is covered by personal liability insurance. Some states have case law which establishes the same precedent. Statutory and case law changes over time so if you rely on this you need to keep on top of any changes. Unfortunately this kind of information can be difficult to obtain.
See
http://www.irmi.com/Expert/Articles/2006/Cooper01.aspx for a legal primer regarding this subject.