Due to the age of the BR-8, Roland no longer stocks zip disks unfortunately. Manufacturers stopped producing zip disks years ago and we no longer have resources for where to directly acquire these. However a good starting point would be to search online:
https://www.google.com/search?client=firefox-b-1-e&q=100mb+zip+disk
Note that the BR-8 was compatible with a maximum of 100mb zip disks. Larger zip disks are not compatible.