Fred at BST Automation is the place for Hi-win and ball screws. Mention the Dean from this forum sent you and you'll get a good price.
https://bstmotion.aliexpress.com/store/314742

One word of caution that 300mm in the Z-axis will significantly weaken the machine so make sure you build it and the Z-axis very strong. Also with ball-screws over 1500mm long you will get whip at higher speeds, so selecting the correct diameter and pitch is very important.