i have recently looked into this aswell, as far as i looked you cannot buy a cable with just a plug and wire. the origional garmin cables are very expensive, so not ideal to cut up.
on ebay there are people who sell compatable cables alot cheeper. these can double up as power leads as they have the car cigggarette plug end on. there is also a serial socket for serial pc connection which i removed to get access to the data wires.
i have yet to connect to my icom m505, so cant tell you wether its actually possible, but i believe it is in thoery!