HorusBennu TG-1 thumbgrip that i just purchased. it was a lot lighter and maybe a bit cheaper feeling than i thought it would be. but it works great, really improves the ergonomics a lot. especially i find it easier to have a smoother shutter release using the thumb grip because of the hard button press on the x100. at $43 US it's a bit expensive for what is a small piece of metal but improved the ergonomics a lot for me. useful tip, when you install the small red pads, take the screw out and stick two of them. unfortunately i have a circular mark on my hotshoe because I overtightened with only one red pad.
there are also some alternative ones that are more pricey:
keger, thumbsup copy $48
the custom x100 lensmate thumbgrip which has blocks off the EV comp dial if that's something you constantly bump into. it grips onto the little EV comp rubber bump. $60
the more expensive custom match technical Thumbs Up EP-5S which looks nicer $108 at popflash though