This one:
http://tinyurl.com/aty49d looks to be a pretty good bet. It's cheap enough, plus only has a 12mm diameter shaft, so smaller, higher speed rated, bearings could be used. I've not seen anything smaller than ER11, but would guess that this would be about ideal for use with 1/8" shank cutters.
I had a real problem finding suitable thin section bearings for my spindle, like you, I wanted to keep the outside diameter down and with a 20mm chuck shaft this was a bit of a challenge. Looking at that small ER11 chuck I'm tempted to get one and build another spindle, because it looks to be a better bet than the one I have.
Perhaps best if I finish one machine first though, before building parts for another..........
Jeremy