Hi Jeanette,
Actually, I got lucky. The place that was supplying me with the high calcite lime, also has marble dust.
Their name is Mineral Specialties, and are located in North Adams, MA.
They've been giving it to me without charge, cause I require a few buckets, while their normal shipment is by the train-load.
You might google "lime" or "limestone" and see if there is someone in your area. But make sure it's high-calcite and not dolomite lime. Dolomite contains too much magnesium, which affects the pigments when you paint.
Harvey