CR1616
Probably a CR1616. Open it up and see what battery it takes.
3 button fob 2003 onwards cr1616
Lithium CR1632 (2008). I just replaced mine and got it from the dealer.
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.
Panasonic CR 1616
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.
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.
This link will show you how: http://www.youtube.com/watch?v=Za7pCdUde3U
the key fob battery is the battey in a key fob torch