What you describe as a 'Bush' is known as a Gib Strip. It is used to remove play in (this case) the cross slide. The cross slide should have a number of set screws that bear against the gib strip that should be adjusted to remove sideways play, but not so tight as to prevent free movement of the slide.
I doubt you will be able to purchase a ready made replacement, so making a new one will occupy a little of your time. The existing one should be able to be used for this job.
Basically, start off with a length of good quality steel and machine off two sides to 30 degrees, so that when it lying as in your photos, the top and bottom are level.
To get an idea of the cross section required, put the cross slide back on without the gib strip and measure the gap (height and width) When made there will be just a few clearance to allow for adjustment and ensure that the new gib strip does not rub on the saddle. It will be a good idea to put some dimples on the new piece where the grub screws bear; these prevent the gib from not sliding with the cross slide.
Dave
The Emerald Isle