Another source is a company called Fastenation, PO Box 1364, Marblehead, MA 01945. I do not have their phone number or know if they have a web site, but we have mail-ordered several times from them in the past. 1998 prices for zinc-plated "ceiling buttons" as they call them, $16/lb (21 doz./lb).
Don't use regular washers, they will stick out too far, the screw head will stick out further. The ceiling button has a built in counter-sink and is relatively flexible. It also has a number of tiny holes that help "lock" the plaster.