That's because they are older models. When they came out, there were all kinds of great covers.

You might try eBay.

Here's a couple on Amazon:

kwmobile Case Compatible with Kobo Aura H2O Edition 1 (12 options available, I like Rainbow Cubes)

kwmobile Cover Compatible with Kobo Aura H2O Edition 1 (1 option)

kwmobile Case Compatible with Kobo Aura Edition 1 (6 options)

kwmobile Case Compatible with Kobo Aura H2O Edition 1 (6 options)

Finding these took grueling seconds of diligent searching.