Stepper upgrade

I want to upgrade the steppers on the x carv. What is the best size and the best place to get them.

Most Nema23 with 76mm long casings (3") should do, low mH (inductance) number is preferred. This will put you in the 270oz/in* range.
A 76mm Nema will not fit original Z-axis without modification.

If you have XController with Nema23 270oz they will be the strongest part in your chain, no need to go for more. If you want to utilize the added power with harder acceleration settings etc a belt upgrade would be beneficial.

If you dont have the XController, getting one or making your own will be a massive upgrade from shield.