Need a bit more information - as vibration sources can come from nearly anything.
Did you buy the car new or used? What kind of service history does the car have? What has the dealer checked until now?
Did you notice anything else, other than the ecessive vibration, at the same time six months ago? Anything added/modified to the car at that time or since then? When is the vibration most pronounced - at idle and stopped, low speeds, high speeds, coasting, in or out of gear, when applying brakes/slowing down, when accelerating?
Unlikey, but still possible that at least one of the motor mounts is damaged or loose.
2002 Corolla S, Silverstream, 1.8L 1ZZ-FE (VVT-i)
2003 Matrix XRS, Cosmic Blue, 1.8L 2ZZ-GE (VVTL-i) - RIP
2009 Matrix XRS, Blue Streak, 2.4L 2AZ-FE (VVT-i)