You can get a new 37-key set (A-Z and arrowkeys) which should match pretty well. The best option is to see if any GH'ers have a steelseries and have replaced the keycaps. They might be able to get you the one.
Another option is to replace it with a compatible keycap from another keyboard. Lots of people replace their CMstorm and filco keycaps. I don't have a steelseries, but I'm pretty sure they all use the "OEM PROFILE".
A final option is to get a single keycap made up by WASDkeyboards. It should be the same profile.
I'd offer to send you mine, but It's part of a dvorak set, and completely the wrong profile and colour.