We use crushed oyster shell here, the same stuff they throw out to the chickens. You can get it at a feed store (if you live somewhere rural and chicken-ey) or at a pet store that sells marine aquarium supplies. Make sure to rinse well and soak before you use. If your water's just a little acidic, it dissolves slowly with every watering, just enough to provide trace amounts.
The stuff's heavy, so if you order it by mail, be prepared for some shipping cost.