I am building a bench power supply (first project). I am having a hard time locating a 120VAC step-down to 12VAC. Whenever I search for a transformer none of them are ever described in such terms. How do I proceed? Thank you.

 

Radio shack sells them regulary, if you want to look them up on the innernet, google " POWER TRANSFORMERS"

They ususally give the ratings as input and output voltages, with there respective current ratings.

 

Instead of building a linear supply, you should consider converting a spare ATX or ATXplus12 form factor PC power supply into a bench supply.

Google "ATX bench supply" for lots of ideas.

I converted an old Compaq 250W supply years ago; has +3.3v, ±5v, ±12v. I have a separate linear supply if I need a variable supply, but the converted ATX supply has proven itself to be quite handy.

 

Converting an ATX supply is a good idea, but I think you would learn more from building a conventional linear power supply: transformer, diode(s), capacitor(s), and a reasonable load resistor. You can consider adding a regulator circuit after the basic unit is working.