I'm currently restoring a S13, while trying to get the ABS sensors out of the front knuckles, both was damages
I have found the sensors in the US for 650usd, but my friend has a Skyline R34 thats a built drift car, with mint ABS sensors that he's not using.
I havent tried to fit it yet, but here is a picture:
http://db.tt/g3mK4YiA
The contact is different, the diameter seems to be almost the same.