One option is to use any bushing set for an 8mm kit that you might have on hand. Check the dimensions with a digital caliper to get an idea of how much more or less that you need to turn down. When you get close, stop the lathe & check your progress until you get it turned to size.
You can also make your own bushings. Any scrap hardwood will work if you don't need to turn a lot of these kits. Corian would be more durable if you need it to last longer.
You could also turn between centers without bushings & just use calipers.