Nitto make some great risers, but they are costly compared to other brands. If you want something cheaper, look into Godspeed or Soma.

As for the size, you need to find out what clamps you have. 31.8 is most likely the thickness of the bars.

As for the width, you can always cut down the bars to make it right for your bike.

The most popular grip for riser bars are probably Oury grips.