Your axle is machined to simply use a Snap ring to limit how far the hub can slide on as the nut is tightened.
You could buy a couple snap-rings at any good hardware store.
I'd probably pick up a couple good/hard flat-washers to slip 1 in between the snap ring and hub on each side too.
(hardware stores would also sell you a foot-long length of keystock you could cut down to the lengths you need, and have some left over for spares)
Take your axle/hubs along to the hardware store so you'll get the right sizes...
This kart shop list the snap rings like you need:-->
http://www.out2win.com/catalog/axleus.html
is a snap ring that strong to keep a tire from sliding??? i might be wrong just wonderen