Budget Bicycles sells tandems. They are located on Regent Street near the university.
If you're interested in a little trip, you can also go to Village Cyclosport in
Oak Grove Village, Illinois (near Ikea). We recently purchased a DaVinci
tandem from Vince, the owner. We were very pleased with his knowledge
Williamson Bicycle, in the old train depot on East Washington, also has a few tandems. Co-Motion lists them as a dealer. But I don't do business there, so don't know how "tandem friendly" you will find the shop.
Sad that such a bike-aware town cannot support at least one shop that specializes in tandems.