It might be helpful to remove the chime from the wall and look to see if the wires go up or down inside the wall. That will give you a clue where to go next.

Thanks for the reply.

I was able to find some wires that matched those coming out of the wall at the chime stuffed up in the ceiling in my furnace closet in the basement. As it turns out, the transformer was attatched to the switch box on my OLD furnace which I had replaced about a month ago. The furnace guys must not have known what it was for and just cut the wires and threw the transformer away with the old furnace.

My house was built in 1995. The doorbell quit working 8-10 years ago. I initially couldn't find the transformer and decided for the time being, it was okay for people to just knock on the door. Fast forward, it's 2015 and I finally decided to get the doorbell working. I have two outdoor buttons, neither were lit. I looked in the attic, crawlspace, closets--everywhere, no transformer. After going online and seeing what others have said about possible locations, I located it underneath the main service panel cover--not inside where you access the breakers; I had to take the whole service panel cover off the wall. One wire had become disconnected from the transformer. Hooked it up--both doorbell buttons lit up and they both work now.

Inside the panel are labels that indicate which breaker goes with what circuit for various rooms of the house. The one for the doorbell was not labeled--so I labeled it. I also took a black Sharpie and wrote the location of the transformer on the inside of the bell cover--seems like that would have been a logical thing for the installer to have done in the first place.