I have a single very expensive stud finder that works on plaster, but it will mess up if expanded metal is present.

Expanded metal was used to strengthen interior corners, hold metal bead on exterior corners, and even as the entire lathe in place of wood strips.

By a couple 1/16 inch drill bits and start hunting.

While nominally on 16 inch centers, older house often wander a little.
Measuring can give you a place to start drilling, but you need to verify the stud location at the exact height you want to anchor.

The lightweight spackling putty makes short work of filling the small holes.

the drills will quickly be dulled, and not be useful for anything else without sharpening.
Save them for future use.
They can be pretty dull and still work in plaster.

The plaster will

there are stud finders that have deep scan and similar modes and that work fairly well in plaster. always double check a result by scanning a couple feet up on the same vertical plane.

the studs will be 16" apart from the center of one to the center of the next. starting from the corner, you can measure out in 16" increments and get a good idea where the stud will be. then you could drill a single hole and if you don't hit a stud, use that hole to find the location of the stud. do this by fashioning a rigid metal exploration tool out of an old coat hanger and linesman type pliers. this can work but you have to slowly rebend the hanger a number of times to get a good idea of distance.

how big is the tv? a big anchor in a plaster wall can hold a lot of weight.

@ civ_IV_fan, we have a 40" TV.

"deep scan and similar modes and that work fairly well in plaster"

Not really.

There are many other things in plaster that make them fail (like metal mesh) and all the 'deep scan' in a $50 detector is not going to help.

Move into the $150 range and you can get some that work more often, but they are still fooled by metal mesh (and fail badly if the mesh is the lathe).

Thank you for the responses. @Brickeyee DH and I did exactly what you suggested and we were able to mount the TV to the wall in the middle of the studs. We got it done on Saturday and it is still on the wall so I guess that's a good sign. Thanks!