Get a better stud finder. Zircon makes several decent models in the $20-30 range; they have a standard and a "deep stud" setting, which can be useful for plaster walls. I just used one to mount a heavy oak stair railing in a house with plaster walls. The stud spacing I encountered was 16" in most locations, but varied, and sometimes there were double studs; I would have missed a number of studs if I counted on consistent spacing. Once I located a stud, I put a piece of masking tape in the location and marked what appeared to be the edges. Later, after I had them all marked, I used a 1/16" bit to drill several tiny holes at each location to make sure there was actually a stud there and where the center was. If your brackets don't cover the little holes, it's a trivial matter to spackle them.

If it turns out to be too difficult to find the studs, I suggest trying some of these fasteners. https://www.homedepot.com/p/FLIPTOGGLE-FlipToggle-1-4-in-20-x-2-1-2-in-Plus-Bolts-25-Piece-425M/310266136 You drill a 1/2" hole in the wall and push the metal brace and plastic link back into the hole. Then pull the plastic link toward you and it will pull the metal brace up into place where it lines up with the hole in the wall. Then insert the screw and tighten.

This type of fastener is sold in a lot of different package sizes (number of bolts), at least two brands, and a few different diameters. This ones I linked to are supposed to handle up to 3" depth for the wall, which I think should be enough for a plaster and lath wall.

Bruce

Bruce-

Those are good fasteners, and I know they are supposed to handle a lot of weight. My preference is always to find the studs, but I think your hardware suggestion is fine as long as the OP doesn't heavily load the shelves...at least with books ;-)