Lithium CR1632 (2008). I just replaced mine and got it from the dealer.
CR1616
Probably a CR1616. Open it up and see what battery it takes.
3 button fob 2003 onwards cr1616
CR1616
A small battery that fits in a key fob or remote.
Open the key fob, remove the battery and look and see what size it is.
Open it up and check and see.
To change the battery in a Lexus RX350 key fob, start by using a small flathead screwdriver to gently pry open the key fob casing along the seam. Once opened, locate the battery, typically a CR2032 type, and carefully remove it. Replace it with a new battery, ensuring the positive side is facing up, then snap the fob back together. Finally, test the key fob to ensure it is working properly.
Panasonic CR 1616
Car alarm in Lexus ES300 started going off at random when car was parked, not running. I changed the key fob battery (flat watch battery) with a new battery and it fixed the problem.
Use a dime to open the key fob battery door on the key fob. New batteries are typically available at Radio Shack. Salesperson at same usually know how to open it.
This link will show you how: http://www.youtube.com/watch?v=Za7pCdUde3U