You can definitely buy individual Lego bricks over sites like bricklink, incase you feel like getting parts for a wrist rest... ![Cheesy :D](https://cdn.geekhack.org/Smileys/solosmileys/cheesy.gif)
Those bricks can also be obtained directly from Lego via the Pick-a-Brick service. It can be slightly more expensive, and the selection is more limited (but everything needed here is available), but it's also usually simpler.
I really miss a wrist rest for my 'dox, I'll really consider such a solution (although I was rather looking for a slightly softer surface). But I'm curious: what did you used as "feets" to avoid that the wristrest slides too easily on the desk?