Dolomite is a type of limestone that is a mix of calcium and magnesium carbonates. It's a bit less soluble then regular old calcite or aragonite (just Calcium carbonate), plus it also provides magnesium.
If you can't get dolomite, I imagine any other source of calcium carbonate should work somewhat- cuttlebone, crushed coral, limestone rock, etc.
Is your source water carbonate weak?
Above 2dKH of water carbonate hardness I don't personally see the need for the additions.
The Fraternity of Dirt
If at first you don't succeed,,, keep kicking it RubberSideDownOnTheLanding, 2-75g planted, 5-55g planted, 5-20g planted, 110g w/30g sump, 8-10g, Refugium, doghouse/newbie 2012 update adding table top pleco pans & a 90g (Nutz)
I also thought oyster shell was CaCO3, not much (if any) magnesium.
From Wikipedia, Seashell:
Because seashells are in some areas a readily available bulk source of calcium carbonate, shells such as oyster shells are sometimes used as soil conditioners in horticulture. The shells are broken or ground into small pieces in order to have the desired effect of raising the pH and increasing the calcium content in the soil