How do I find the studs behind plaster walls?

Quit speaking to Zircon- they are trying to rip you off. I can find the studs with my knuckles as well as their videoscanner.

If you find the nails, you have found the stud. You can purchase any type of stud locator at any hardware store, from a simple magnet to electronic. They all work on the same principle- locating the nails which are in the studs. You can even rap on the wall and you will hear a hollow sound until you hit a stud.

I wonder if ganache has expanded metal lath rather than wood lath. That would be enough to send any magnetic device into a tizzy.

If all else fails...Measure 16 inches out from the corner of your closet. Get a small diameter drill bit and start making a few holes in a horizontal line about 1 inch apart going in both directions. Make the holes right behind where the shelf will be if you don't feel like spackling them up. Eventually you will hit a stud. Once you locate a stud the others will be either 12, 16 or 24 inches apart.

Is that the AMAZING ZIRCON?

I bought a stud finder-called my ear and fist. I too was going to install a closet organizer and have plaster walls(what a mess). I do have a battery operated stud finder-but found it was only confused by my wood lath backing for the plaster. Maybe it was a cheap finder who knows. Listen for a solid knock as opposed to a hollow or shallow sound. Put a mark on it, continue across the wall until you hear another dead sound, mark it and measure between the marks. that measurement tells you where your studs will be. Oh, the other thing I found- get the brackets that support the shelf. My shelf fell out of the wall- only for me to find the studs that I drilled into had been attacked by carpenter ants. Not fun!!

There are three ways of finding studs:
Electromagnetic, acoustic, and physical.

Those little magnet gadgets work as well as
any electromagnetic thing, but they will find
*any* steel or cast iron, including pipes and
conduits, not just the nails in studs.

Acoustic thumping around will get you sorta
close. One easy variant is to wrap a
cordless electric shaver in a rag, and run it
across the wall. It gives you an idea where
the solid and hollow sounding places are.

The best stud finder I ever had was called a
"Morrison Probe". it consisted of a little
plastic triangle with a curved tube attached
in such a way that you could insert the tube
thru a small hole in the wall, and the
triangle would hold the tube so that it made
a gentle 90 degree bend inside the wall.
Then you'd insert a piece of stiff piano wire
thru the tube, holding it horizontal, until
it hit the actual stud.

Since I don't have that great gadget any more
I'd use the shaver trick to get close, drill
a little hole, and probe over with a piece
of #12 copper electrical wire to find the
exact edge.

Sometimes you'll come upon the place where
they stopped measuring from one end, and
started from the other end, so the distance
between studs for one space will be smaller
than the constant 16 inches or whatever it
was.