I know this wont help the person who lost theirs, but it may help the rest of us. The car manual says on page 3-5 under "Consumer Key programming": "You can program new keys to the system if you have two valid (already registered) keys and blank (not registered) immobilizer key"... It goes on to say the key must be cut for your vehicle. Then it gives the programming procedure.
My Remote key I have says this on it (you must open the black plastic on the key shell to see it):
FCC ID: OUCG8D-625M-A
OMRON Corporation Japan
The key blade is a "MIT3" (be sure the key blade matches yors in any photos of the blank).
There are several on-line places including e-bay sell the key remote (G8D-625M-A):http://www.ebay.com
I'm going to try it out and see if it works. That's assuming I can find a locksmith to do the cutting on the key.
October 2ed, 2012 - Update:
The key advertised on eBay did NOT work there is a tiny difference in the key blank. The locksmith cut the key for me but it wont take the programming as described in the manual. I notice the key blank (from eBay) has more blank space between the black plastic and the cut on the black. Also the locksmith said the encryption may be different on the i-MiEV than it is on other Mitsubishi models. He is going to see if he can find the right one. I won't hold my breath for that. So I ended up with an extra remote which is not programmed. The key is not good otherwise.
John - 2012 Silver i-MiEV SE model, Jan 19th, 2012 w/OpenEvse, caniOn,& OVMS.