I have an old house built in the early 1930s. The conventional stud finders are useless on plaster. The more sophisticated ones are more expensive in the $50 range.
What features would a stud finder need to have to work well in my situation?
This stud finder with magnet and level is what I keep on me at all times. It is the only stud finder that I have and works excellently. It is strong enough to pick up nails used for crosses most of the time too.
I have no affiliation with the product - I am just a customer that has bought 6-7. The magnet is super strong and the pointer + level help. The only downside is the plastic tip breaks on every one if I drop it, but it still does the trick. A bonus is that it finds screws, nails, tools, whatever... in hard to see/reach places. I don't know how many times I was up in an attic and dropped something and pulled out the stud finder.
If the house construction is lath and plaster (with no metal mesh), you may be able to use a stud finder with metal scan capabilities or a dedicated metal finder. The reason why these work, over basic stud finders is because metal finders work by finding the nail heads that attach the wood lath to the studs.
If metal mesh was also used to help secure the plaster to the lath in some instances, even metal finders may not help you find the stud.