California Sidecar Friendship II. United Sidecar Association has a for sale section... they have great forums too, and it's also a good place to look for sidecars for sale, and complete rigs: http://www.sidecar.com/default.asp
To add to the great advice you've already received, any big Goldwing makes a great tug IMO. Getting the steering modified to reduce trail and steering effort is very important though - I was amazed at how much heavier my steering became with the sidecar fitted... and it is set up perfectly. For city driving up to about 40-50 mph it's totally neutral, but at highway speed, due to the added drag from the sidecar (and crown of the road) it pulls slightly to the right (normal). You have to set 'em up for whatever compromise you want, and/or use an electric actuator setup to control leanout of the tug (usually by raising/extending the sidecar suspension).