if it really is custom sized then I know Chris King make custom headsets.
but it'll be costly!
edit: I know how you feel.
The Ti bike I got for a mega bargain turned out to be a massive hassle as no headset looked like it would fit.
but eventually I found one that did, even though it meant I had to get a converter from 1" threaded Shimano 105 to 1" threadless.
is it bigger than 1 1/8" ?
if it really is custom sized then I know Chris King make custom headsets.
but it'll be costly!
edit: I know how you feel.
The Ti bike I got for a mega bargain turned out to be a massive hassle as no headset looked like it would fit.
but eventually I found one that did, even though it meant I had to get a converter from 1" threaded Shimano 105 to 1" threadless.