The general answer is, anywhere that sells tires. One thing to keep in mind is whether or not the place you buy tires from has any lifetime service (free rotations, patches, etc.) or other roadside assistance available to augment any you may already have. Once you know which ones you want to get (refer to this FAQ for some info on that), check around to see who has it in stock and who will install them.
Club members have had positive (and sometimes had negative; we offer no endorsement) results from the following:
- TireRack.com (which may not necessarily install them if you order online)
- Radial Tire Company in Silver Spring, MD
- National Tire & Battery Company (note, for Chill members and above, there is a member discount available)
Additionally, some members have said good things about Pull-Up Tires where you can either order tires from them or buy somewhere else and ship to them, and they come to you. Kind of like Tesla Mobile service. A Tesla Service Center may offer the same service, if you ship tires to them and go there (although confirm with anyone you’re going to ship tires to first before doing so, so they aren’t puzzled when new tires show up).