Try going to a local airport that has a pilot shop, and ask them for a power cable for a Garmin 295 aviation GPSR. It's pretty much the same thing. If you are lucky, they'll have a cable with bare wires on the end - the black is ground, the red is the 12 volt supply.
If they don't have a cable with bare wires, just get one with a cigarette lighter plug, and cut off the plug. Same wire colours. The cable with the cigarette lighter end on it is shorter than the cable with bare wires, you might need to splice in an extension to reach your accessory terminal or battery on your moto.
There are two types of round plugs that fit into your SP III on the end of these power cables. The very early production cables (ca. about 2000 or so) had a two-piece plug with a tiny Phillips screw in the top, and shrink wrap around the end. This one is not waterproof and therefore not ideal for a moto, though I used one for 30,000 miles on my moto before it failed. Cables made after the beginning of 2002 had a single piece molded connector, which is waterproof.
Note also that the cable that comes with the bare wires has a heavier gauge rubber cover around it - it's intended for moto use - but the one with the cigarette lighter plug on the end is a thinner cable, not designed for moto use. You can get away with the thinner cable on the moto OK, just be aware it is not as durable as the thicker one. If you give it a really hard yank, the wire might break inside the rubber cover.
PanEuropean