From LinerWiki

Backrests

There are 2 manufacturers making backrests for the OEM seat:

• BAKUP - http://www.bakupusa.com/YAMROADSTRAT.htm
• Utopia - http://www.utpr.com/roadliner.html

Note - These backrests may require some modification (drilling holes, etc).

The other option is to purchase a new seat with the corresponding backrest. The following manufacturers have seats which fit the 'liner AND support backrests:

• Corbin - http://www.corbin.com/yamaha/linermenu.shtml
• Mustang - http://www.mustangseats.com/yamaha/roadliner/roadliner.shtml
• Ultimate - http://www.ultimateseats.ca/yamaharoadliner.html