IM not sure but i can tell you a good site to get some free onesthat have good sound quality. Go to www.myxer.com. I also had trouble getting my old ringtones so i just decided to get it through that site...hope that helps
The only way you can get your old ringtones to your Instinct is if you purchased them from Sprint and they haven't expired. If you purchased them more than 90 days ago they probably expired and you won't be able to re-download them. If you go to the Web tab, and select my account, it will take you to you account homepage. On this page there is a link for your Content Manager, if any of your ringtones haven't expired, you'll be able to re-download them from here.