The special product cannot be purchased separately.
Dolphin back rest is an universal design for Dolphin leaning post and Dolphin 80L cooler & seat, you are free to attach it to either the leaning post or cooler seat. The angle of back rest is freely adjustable when you attache it to your leaning post or cooler seat, you can test which angle you prefer and find the position where you need to mount.
Above picture shows the back rest with a leaning post.
Above picture shows the back rest with a cooler seat.