The first thing I'm going to assume is that when you ask about buying rental car insurance coverage, you are speaking of a collision damage waiver or loss damage
waiver for the vehicle itself and NOT liability insurance. (If you are asking about basic public liability and property damage insurance, it's probably not something you need to buy; for North America most states and provinces it is generally included in the rate up to the minimum limits required by law (California is a common exception) ...see this page on Liability Insurance. )
International car rental is different and you must check insurance info specifically for the city and country.
Ok, back to your question. The answer is Yes, you can purchase insurance for a rental car from someone other than the rental car company - you can often buy it from companies that sell travel insurance.
The main thing you need to know is, that while it is a cheaper per day price than the rental car insurance coverage offered by the rental company, it is not quite the same, nor offers the same amount of coverage.
I don't believe that when you purchase insurance for a rental car, one is better than the other, just that they are different and you just need to be aware of all facts in order to make an informed decision.
There are different sites or companies that offer different types of trip insurance coverage, including rental car insurance coverage. This answer will focus on one such company, only for brevity. I encourage you to research other such options and make sure to check all terms and conditions of coverage before you purchase insurance for a rental car.
That being said:
I did some research on rental car insurance coverage through TripInsuranceStore.com They offer a CDW Plan for $9 a day, plus a one time $3 service fee (note: price and info below accurate at the time of this response.. prices and details below may change - check the website to be sure).
Certainly a much better price for you when you need to purchase insurance for a rental car! Here's a breakdown of the major differences between this coverage and the rental car insurance coverage offered by the rental car company.
The tripinsurancestore.com CDW plan does NOT apply to certain vehicles.
The rental car company coverage would include any of their vehicles.
To qualify for tripinsurancestore.com CDW plan, you would have to be renting AT LEAST 50 miles from your home.
The rental car company coverage would cover the rental no matter the distance between where you live and where you rent.
With the tripinsurancestore.com CDW plan there is a $250 deductible
With the rental car company full cdw coverage, there is no deductible.
You can purchase insurance for a rental car from tripinsurancestore.com CDW plan in advance of your trip, before you sign the Rental Car Agreement.
The rental car company coverage would only be available to purchase at the date and time of pick up of your rental.
With the tripinsurancestore.com CDW plan, you have to purchase to cover all the calendar days you have the car - even partial days. for example if you picked up the car on Nov 1 at 12 noon and returned it on Nov 8 at noon, you would be charged for each calendar day --- Nov 1, Nov 2, Nov 3, Nov 4, Nov 5, Nov 6, Nov 7 and Nov 8 - 8 days @ $9 = $72
With the car rental company coverage, you would be charged for 7 - 24 hour periods if you purchased the $20 insurance. = $140
The tripinsurancestore.com CDW Plan does not cover theft or stolen vehicles.
With the rental car company coverage, if you purchase LDW, this covers theft (loss). If you purchase a CDW, it will only include Damage (collision), not loss (theft)
If you look into this option to purchase insurance for a rental car from tripinsurance.com , make sure to check the "certificate of coverage" link for the state that applies to your situation.
If you plan on having additional drivers on your rental, you would want to check if this option would carry over if they were driving as well (the rental car insurance coverage from the car rental company covers the renter and all authorized additional drivers).
You may want to check also, with your car rental company of choice as well as with this provider, of the "turn around time" for coverage. By this, I mean, how quickly will the car rental company charge you for damage? Is it charged to your credit card? Will they bill you and be willing to wait until the claim goes through to be paid for damages to the rental car?
Depending on the type of trip that includes your car rental (for example if your trip involves airfare and accommodations as well), you may also be able to get rental car collision insurance as part of a complete travel insurance package. As we've already discussed, make sure to check all terms and conditions of coverage to make sure that it fits your car rental needs.
I hope this helps!