Whiteline has discontinued it's rear anti-roll (sway) bar, part number: BNR28 for the N16/B15/G10. The reason behind the discontinuation is a lack of demand :(
For Slowtra owners looking to upgrade your rear arb, your choices are from what I know, limited to Ultra Racing, Eibach and Progress Auto. Apart from Ultra Racing, non are available locally and both the Progress and Eibach ARBs require drilling your rear beam trailing arm. All rear arbs/sway bars are interchangeable between a N16, B15 and G10.
Progress Auto hails from the USA and uses a 22mm bar (Part number: 62.1511) similar to the Whiteline arb. The Eibach rear arb is only 19mm (Part number: 6362.312) but also has a front 25mm arb specification for the USDM Sentra B15. Part number for the rear arb is 6362.310. You can also buy both the front and rear as a complete kit (Part number: 6362.3200). Ultra Racing are from our very own Bolehland and has 2 rear arb specs for the Slowtra N16 - One is 16mm (Part Number: UR-AR16-039) and a bigger 19mm (Part Number: UR-AR19-043). They also have a front 22mm arb application (Part Number UR-AR22-343).
I could be wrong but I believe Ultra Racing still uses rubber bushes for its Sentra arbs while Eibach and Progress Auto uses polyurethane bushes. Long term reliability of the UR arb is also possibly a suspect but the upside is the UR arb cost significantly less than what you would pay for a Whiteline (when available), Eibach or Progress Auto. Your call..