• DupeShop (Australia|Melbourne) - Bought blanks and cases from here with no dramas. Sell new ones in various lengths and styles, and used ones.
• Otherwise I've just bought old tapes from op-shops and recorded over the top.

Dimensions of a J-card are about 10×6.5cm for the image,

If you have a laser printer and some nail polish remover (acetone) you can transfer stuff from paper to the cassette: Custom print on your cassette tapes - DIY

This guy does it with a rubber stamp and solvent ink: D.I.Y. Cassette Printing Session

• Cassette tape