I know of only a few that are actually available beyond the one offered by BMW. Post the links of you know of any others.
Roland Sands:
Motorcycle BMW Products | Motorcycle Parts and Riding Gear | Roland Sands Design
Corbin:
Corbin Motorcycle Seats & Accessories | BMW R Nine T | 800-538-7035
Wunderlich:
Sitzbänke & Zubehör - Ergonomie & Komfort - NineT - BMW R-Serie
Roland Sands:
Motorcycle BMW Products | Motorcycle Parts and Riding Gear | Roland Sands Design

Corbin:
Corbin Motorcycle Seats & Accessories | BMW R Nine T | 800-538-7035

Wunderlich:
Sitzbänke & Zubehör - Ergonomie & Komfort - NineT - BMW R-Serie