If you want to customize your Freestyle Edge with individual keycaps we think Max Keyboards in California has the best selection.
You can order custom-legended keycaps with your own artwork starting at $7:
If you just want some basic blank keycaps that will let the backlighting show through, these sets are available starting at $6.99 for 9 keys (note that only 3 of the 9 are Row 2/C style standard on the Edge).
Signature Plastics: http://pimpmykeyboard.com
WASD Keyboards: http://www.wasdkeyboards.com/
Let us know if you have other favorite vendors and we'll add them to the list.