I got quiet a spares box too.
Many of the Kits I build come parts that are not needed for that version, etc guess where they end up.
Earthquakes over here at times knock a few Models of he shelf, what can't be repaired/fixed goes in as well.
Granted some of the spares in there are actually AM sets, Supplies(Plastic rods, etc) and leftovers from them.
Be patient and inspect all your sprues and not used parts like decals, etc. soon the spares box will start to grow.
Also visit fleamarkets and similar and buy cheap old Kits to add to it.