Ringback tones are not stored on your phone but they are part of the infrastructure of your service provider.
So if you are on e.g. verizon you can get them through verizonwireless. But you'll have to pay a one-time activation fee as well as a monthly fee to use the service.
read up more on ringback tones here
. Maybe you'll have to scroll down a bit, to the article called 'The most common misunderstandings about ring back tones' (right now it is the second post). Their blog is a bit dodgy somehow.