I actually got them from a friend... I believe he got them from a local parts/hardware store, but I'll find out. I would like to pick up some more, they are the ****.
The specs of the axle nut I believe is M20x1.5 for thread pitch with a height of 18mm and width of 32mm.
I have looked all over the internet and I don't think anyone makes this size nut in slotted or castle slotted format. It seems like it would have to be custom made.