Here it is:
If you purchase the phone from a 3rd party retailer, that is your source of aid for the first 30 days of your contract, or the "trial" period. After the 30 days, your main source of support is Sprint stores. Because they use the same contract (sprint contract) you still have everything the Sprint store would give you. Think of buying at best buy as a Sprint store inside best buy, sam with RadioShack.
I do not recommend any of the 3rd party insurance plans, as they are usually less efficient, and will not cover loss or theft, as well as accidental damage (such as water damage, and dropping). There are some insurance plans that are sold by best buy and radioshack that will include accidental damage, but remember that those plans are through another 3rd paty company. So a third party of sprint uses a third party to handle insurance, which then uses a third party to get the phone. The result? Longer wait times. With Sprint's insurance, you can call Asurion, and your phone will be delivered the day after. That is not the case with others.