The Motor Vehicle Tentative Credit can be found on the manufacturer's certification letter to the IRS. If the car is foreign, the domestic distributor's certification can be used. The manufacturer or domestic distributor can provide the purchaser with a copy of the letter acknowledging the certification for the vehicle.
The Alternative Motor Vehicle Credit has been extended to include tax years 2019, 2020, and 2021. If you have a vehicle purchased in 2017 but not put into service until 2018, you may still be able to claim the credit. Refer to the 8910 Instructions for more information.