I'm sure there are lots of ways to do this, but the simplest is to go to Verizon's website and look for the downloads link from the home page. That way when you choose something, they just charge your regular cell phone bill for it instead of dealing with paying seperately from another site not associated with your provider.
The download page will be the area where you can find all the ringtones they have available. You can search and see what all is available and choose your favorite.
They will ask you to enter your cell phone number and send it to your phone. When you get the message (takes just a second to send) you can save the ringtone to your phone.
You will probably need to go in and "choose" that song in your set-up menu and then be good to go.
Hope that helps!~
I'm going to brave the world of uploading my own thanks to this thread!