Sprint offeres content for your phone through the "Fun" option from your home screen. Then "My Stuff", and you'll see the categories for content for your phone, one of which is ringers. There should be a "get new" option once you've selected ringers. Search for a song or artist, click "buy" agreeing to the price. You'll get a text message with a web address allowing you to click on it to download your content.
You can also try a 3rd party provider like myxer.com // It may have some free content like ringers, but Sprint doesn't support it because they're not affiliated with myxer.
If I don't have a data plan, do I then pay data charges if I access the ring tones through my phone and download them?
yes you will be charged data and for the ringer unless its free. But its not much atmost a buck or two.
If you don't have a data plan you will be charged $.03kb for data even if the ringer is free, Depending on how many kb's the ringer is will determine on how much you will be charged, It averages from about $4.50 to $7.50+ in data charges.