That's weird. I looked through all the major (non-oem) auto parts stores online and seemingly none of them can find a battery for the 2016 Scion iM.
I searched for the 2017 Toyota Corolla iM, and was given various results, but Size 35 batteries are the most common. I'd start with that, I guess.
I know right? Was in Walmart and they have a terminal where you can look up your car and it will tell you which battery is compatible. No Scion iM but 2017 Toyota Corolla iM was there. Said the same thing - 35N ($215 CDN for the AGM).
Wonder how much it would be if I just went to a Toyota dealership parts desk and bought the battery there.
Just to close of this thread... I ended up buying a Toyota battery from my local Toyota dealership for $157 (simply walk up to their Parts & Services department).
The battery is a non-flooded group 35 (size), cold crank amperage (cca) of 550 and reserve capacity (rc) of 90 minutes. Any battery matching these specs will do.