Those axles are very difficult to find.
I once created my own conversion axle from off the shelf components.
Here's a guide to dissembling the hub:
Hub How-To - Zipp 182 - Slowtwitch.com
The stock bearings are 6802 which are (15 x 24 x 5mm) and the zipp track axle is 15mm diameter to fit these bearings. The axle hardware I used was for a bearing with a 12mm ID so I swapped the 6802 bearings for 6901 bearings (12 x 24 x 6mm) on both sides. The center sleeve you see in that tutorial won't fit the different bearings so leave that out.
The hub hardware I used were spares for a Miche primato rear hub. I used the 10x1 solid axle, nuts, "cones" and lock nuts. You'll also need to some axle spacers to get the spacing and position correct. Most bike shops will have a bin of hub hardware where you can find some spacers.
Later I found out my friend had the Zipp conversion axle and he wasn't using it so I bought his and swapped the bearings back in my disc so I could easily switch back and forth from road to track.
Another solution would be to have a knock off made by a machinist. If we could get enough people together that were in need I could probably have some made.