Where to Buy Keycaps
Modifier Key Keycaps
Front-Side Letter Keycaps
These are keycaps with all sort of icons on them.
You can find more at
Artistic 3D Keycaps
Keyboard Keycap Gallery
Keyboard Keycap Gallery 2
The artistic 3D ones are typically custom crafted, not mass produced.
They can be expensive, $30 per keycap or more.
You can also find them on ebay, massdrop, or via keycap trade from reddit or geekhack.
What's ABS, PBT?
Keyboard Keycaps: ABS, PBT
Keyboard Key Label Printing Technologies