Go-kart axle recommendations?

Hey there!
I’m currently in the middle of building a custom go-kart, but I’ve hit a bit of a snag regarding the rear axle.
I’m using some wheels from an old TaoTao quad, and my plan for the axle is to use a heavy-duty thick-walled pipe, then slide the original TaoTao axle sleeve right inside that pipe to reinforce it.

https://postimg.org/image/ryfm5nl6d/

That’s the axle I'm talking about.

What I'm trying to figure out is where I can actually pick something like this up, or even what the technical term is so I can hunt for one on Craigslist or somewhere similar.

Thanks!

If you're talking about that jagged little section right there, that's called the knurl.
It’s usually just some industry standard, but now we gotta deal with... what, inches or metric?