adanecsleds.com has a very good selection and can even make custom harnesses that are better fit for the longer body of a shepherd. I believe they sell the lines and connector that you can put on a bike in order to make sure the lead lines clear the front wheel.
I'd look into a sled dog club in your area. The one in my area is pretty chill and they allow pretty much any dog to come with them. It's a much better way for dogs to learn a new activity because the lead dogs guide the new dogs. If you've been teaching your dog to heel its whole life...its quite the concept to get them to actually pull YOU on a bike since you can't actually be out in front of them getting them to go faster or to actually accept the tension on the harness.