There are a couple Minn Kota forums on the Internet. You could try posting a question at them.
The other thing you could try is calling Marine Electric Service in Hendersonville. That shop is an authorized service center for Minn Kota and they can be pretty helpful for finding obscure stuff.
However, if I was guessing, I'd say you are going to have a tough time finding that part. We're talking about a part that isn't made anymore for a motor that's 15 years old. Even if you go through the trouble of hunting down this relay and replacing it, how long will it be before something else wears out?
If you watch the boats section on Craigslist closely, good deals pop up fairly regularly on used trolling motors. It's easier in the spring than in the fall, but you'll find something if you're patient. I could be wrong, but I think you'll be happier in the long run just replacing the motor.