Shifters compatible with: SG-3D55 (Shimano Nexus 3 speed - disk brake - internal gear hub)
User6527 is wrong there. You can not use derailleur shifters due to the different cable length that is pulled or pushed to shift to another gear. You can not use other brands' shifters either for the same reason.

The shifter you mention yourselve is available in black and silver, and both with and without integrated brake lever. There is also a 'cheaper' version: SL-3S41E.

I thought there was a retro alternative as well, but I can't find it. Not online, and even not at our bicycle parts wholesaler.