I was wondering about getting a longer Z Axis extrusion rail. I went to the store and looked under extrusions. I didn’t see one.
If you follow this link, you’ll see other’s upgrade adventure plus links to upgrade parts.
Do a search for Phil’s Linear Z Axis and you will see what he did.
If you are interested in going with a longer z rail I recommend taking a look at a c-beam kit or one of the linear rail kits from e-bay.
They are a lot stiffer than the current z axis, and making it longer tends to increase any flex issues you may have.