Can you post a picture of the lathe?

Linearmotionbearings2008 on eBay is the cheapest place to easily get ballscrews, plus he does custom end machining exceptionally cheaply.
Look on eBay for the stepper drivers. You want to get 75-80V drivers, so not m542...should be around £40-£45 each.

If you're struggling for room on the Z-axis it's common to put the ballnut on the end of the slide at the back and have a longer ballscrew, like I did here.