Hey Guys
Ski Bike conversion kits are available here:
http://www.skibikefun.com/partsandaccessories.htm
Not cheap but is riding ever?
These are not being made any more and the patent owner is not allowing anyone to manufacter them. If people get in as a group you may be able to get a discount/reduced shipping.
Just thought I would let everyone know as I spent a long time looking for some of these.
They also have adapters for 20mm axles.
Cheers
Cam