How do I hang a pot rack in a plaster ceiling?
You're already putting lots of holes in your ceiling, which will have to be patched.  As long as you don't mind some holes, try this:

Use approximately a 1/4" drill bit to drill into a likely spot on your ceiling.  If it hits wood all the way through, you've just found a joist.  If it goes through the lath and hits a void, get a piece of wire or an old coat hanger.  Put two bends in it so it looks like three sides of a rectangle.  Make sure that the top and bottom sides are the same length.  Work the top side into the hole.  Make it "turn the corner" in the hole so that the top piece of the rectangle ends up parallel to your floor.  Now spin the wire until that top section of the rectangle hits something and stops.  If you made the bottom section the same length as the top, then the bottom section will be pointing right at the location of the joist.