How can I locate the doorbell transformer?
Typically, the doorbell transformer will be on or near the main service panel.

This is usually, done to save money on wire. If the transformer is mounted to the service panel, you'll only need about a foot or two of 14/2 cable to connect it (which can usually be reclaimed from scrap).  The rest of the wiring from that point on will be cheaper low voltage cable, since doorbells typically run on 24V circuits.