The HDD Ribbon is what connects the hard disk drive to the iPod's Logic Board (for the iPod Video and iPod Classic). This ribbon is also required for connecting iFlash boards and other flash mod boards to the iPod motherboard. Two notable exceptions to this include the iPod Mini, which just uses the same connector as a Compact Flash (CF) card (albeit attached to a different type of ribbon cable), and the 6th generation 160GB iPod, which uses a CE-ATA interface instead of the ZIF one featured on the HDD Ribbon. Also, 4th Generation/3rd Generation iPods need the 4th Gen iPod to iFlash Converter to connect to an iFlash board.
One maker of replacement ribbons come from popular flash mod board manufacturer iFlash.
Resources
iFlash's webpage for replacement HHD Ribbons: https://www.iflash.xyz/store/hdd-ribbon/
iFixIt's guide on how to replace the iPod Hard Drive Ribbon (iPod Classic): https://www.ifixit.com/Guide/iPod+Classic+Hard+Drive+Cable+Replacement/565
iFixIt's guide on how to replace the iPod Hard Drive Ribbon (iPod Video): https://www.ifixit.com/Guide/iPod+5th+Generation+(Video)+Hard+Drive+Cable+Replacement/609