Tirerack is also a great source for i3 tires. Similar prices to Costco, and free overnight shipping to most places in the USA. The even have mobile installers in many cities, who come to you with a fully equipped tire-shop-in-a-van and will install them in your driveway (as a bargain rate compared to other shops).
And a little searching will pull up other owners who have opted for totally different tires, using spacers etc. for a good fender fit.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 19 inch 427 wheels, EVoInnovate EVSE