Giant Overdrive 2 fork replacement
You can just buy a standard tapered headset. Or if you can firnt it, only the upper assembly for 1 1/8". Giant uses the same diamete for their 1 1/8" and 1 1/4" headsets as it says here: http://www.pinkbike.com/news/Giant-OverDrive-2-Steerer-tube-sizing-2011.html.

Another solution I can think of is to turn a spacer yourself (or have it done by someone) to work as an inner race for the bearing. It's more demanding (since you would have to turn both the spacer and a cap), but could probably work out a bit cheaper than the entire headset.