Linearmotionsystems on ebay have ball screws and nuts up to 25mm. They can pre machined to your measurements and posted to your door cheaper than what you can buy them locally. Some of the local stuff getting sold, probably comes out of the same factory anyway.
I bought 3 lengths of 25/5mm and am happy with them. I went with 2 nuts to preload against each other, but he has pre loaded nuts available now.