SR-22 insurance actually does not exist. The SR-22 is a form that most states require a driver to file to show proof of financial responsibility. In other words, proof that they have sufficient insurance as required by their state. If you need to file an SR-22, your insurance company will do that when purchasing your car insurance policy.
There are several reasons why a driver may have to have an SR-22, like a DUI charge, driving without a license, or having a long lapse in insurance coverage. If your state mandates you to file an SR-22, you usually will have to do so for at least three years.
Many car insurance companies will sell you an insurance policy and file the SR-22 for you, but not all carriers will. Rates may vary greatly between SR-22 carriers, too, so you should compare rates to find the best option.