I contacted David Silver Spares to replace mine in April last year. There are some on E-Bay but I prefer the OEM key look rather than an after market one.
They supplied the part number after I emailed them a photo of my key. I've just done a check and copied the info from the site. I doubt if the postage would be that great as it's something that might go in a normal envelope.
35121-KW3-771 KEY,BLANK (NO.1) *B
Part number 35121-MT3-670 has been superceded by Part number 35121-KW3-771
Price£7.50
(£9.00 inc vat EU)
Approx€ 8.75, $ 11.30, ¥ 1168.16
Quantity Stock 10 in stock - Usually ships in 48 hours