You can get LED's lots of places online: The electronics sellers will have a truly huge selection (like over 100,000 choices) RS-components, allied electronics, farnell are all places I haven't used (also, aren't some of them the same? like element14?) I have experience with Mouser and Digi-Key.
LED's are constant-voltage devices (well, pretty constant, it will vary slightly with current and temperature) so you want to get LED's that have a similar forward voltage drop. If they drop different voltages, they'll draw more (or less) current, and be brighter or dimmer. If it's too bright, it might not be possible to power the array with the power from USB. if it's too dim you won't be able to see them properly.
Also make sure you get the right size. I haven't measured, but they look like standard 3mm LED's.
It'd take a bit to switch them LED's At the best, you'd have to desolder them all, and resolder them in. I don't have one, but a hakko 808 should help here (other than that standard desoldering rules apply) At worst, you'll have to unsolder the switches and take the switch body apart. This would be a pain.
