The Deda Zero1 is probably the best bang for the buck 31.8 stem out there (Deda labels them as "31.7" but it is perfectly fine to use with any 31.8mm handlebar).

When switching to risers it's usually a good idea to go with a longer stem because your hands will end up being a little closer to you. I switch back and forth between risers and compact drops often and use a 100mm with drops and 110mm with risers.