So collating the info:

J-Cards can be designed at http://www.minidisccover.com/ either works great or is a PITA depending on your luck

Cases can be bought from eBay or https://www.retrostylemedia.co.uk/product/clear-minidisc-case

Labels can be designed here

Print labels on this paper from JP, this paper from USA, this paper from UK

All we're missing now is the perfect paper to print J-Cards on and links. Does that look good? Could we put all this together as a sticky?