The always-popular WeatherTech floormats are popular amongst BMW i3 owners, and can be purchased on Amazon for approximately $150
Another option, the Broadfeet Custom Floor Liners are hand cut and made in the USA. One forum member says the “look way better” and are “waterproof, easy to clean” custom fit mats. [source] They went with the Black & Gray color and they “look really good.” Here are the mats on Amazon.