They are everywhere. Salvage one from a small plug-in LED bedside clock, or other small plug in electronic appliance at a thrift store. You can get "doorbell transformers" (usually about 10V AC) at any home/building supply store, too.
The old style, heavy "wall warts" have step-down transformers in them too. There are buckets of those at thrift stores. Look for AC output, or take apart a DC wall wart and remove the diode and capacitor.
FYI, doorbell transformers are loosely coupled ("soft"). They are designed so the current is limited, so in the case that there is a short circuit, it won't be a fire hazard. Honestly, I can't remember the details, but I believe it makes it a poor choice for electronics since the voltage can droop a lot under load. All transformers do it, but I think they do it more.