There are some premade ones in Cydia under the sounds category... Or make your own and SSH them into your iPhone.Which Cydia app would you recommend?
This guide will show you how to create and apply your own custom ringtones.
It will cover: Converting your file to the right format and applying it.
Converting Your File
The file format of the SMS ringtones is .caf. A .caf file is really an .aiff/.aif file renamed.
The file can be any length, unlike the regular ringtones.
1. Open Your File in iTunes
2. Go to Edit>Prefrences>General>Import Settings and change the 'import using' option to "AIFF Encoder" and exit this menu.
3. Right click on the file and click "Create AIFF Version." This will convert and add your file to iTunes.
4. Find the converted file and right click, then click "Show in Windows Explorer."
5. Once the file is located in Explorer, rename the file to "sms-received1.caf" (You'll find out why in a second) and yes, the file name is case-sensitive.
Applying Your Ringtone
The iPhone ringtones are located in directory /System/Library/Audio/UISounds/ and are named sms-received1.caf, sms-received2.caf,...
"sms-received1.caf" is the ringtone that is set by the phone. When you change your ringtone, it actually just renames these 6 files. In the Settings app, the names you see are actually the 6 .caf files, but when you change one file, the name "sticks."
1. Open up WinSCP and navigate to /System/Library/Audio/UISounds/
2. Rename "sms-received1.caf" to "sms-received1.old"
3. Then copy over the .caf file you created before.
And you are done. Easy as pie.
As a side-note, only those 6 files are read by the phone, so if you create a "sms-received7.caf," the phone won't recognize it.
Any issues, comments, add-ons, or the always-needed help - feel free to reply.