HomeshopCNC should be able to sell you a 5/8 ballscrew at the size you need. I think they charge 1.20/ft.
5/8 might not be the ideal thing for a 5ft length though.
So check Mcmaster.
And make sure whatever you use is well supported on both ends. Use angular contact bearings on the end where your timing pulley or whatever is at, and radial bearings at the other end