Stepper motors can be found in almost any piece of electro-mechanical equipment. From my personal experiences, good sources for stepper motors include:
Surplus dot-matrix printers
If you find one of these at a swap meet, surplus store, or garage sale for a good price, buy it! They usually contain at least 2 motors, sometimes with optical shaft encoders attached to the motors! Also a good source for matching gears and toothed belts. As a general rule, larger printers will have larger, more powerful stepper motors in them.
Old floppy disk drives
These usually contain at least 1 stepper motor, and if you're fortunate, possibly a driver IC that can be salvaged and re-used in your own projects. Along with the motor you will get some optical interrupter units used by the drive to sense the state of the write-protect tabs and to index the disk.
These places buy surplus from others and sell it to the public, often at great prices. The average price for a small to medium stepper motor is usually around $5.00.
Mail Order Companies
You can find surplus motors or even new, packaged units. Naturally the new units are going to cost more, but this may save time and money if you're building equipment with the motors that will be used at more than a "hobby" level. For general tinkering and small scale robotics, used motors will work just fine.