I get mine from fish stores (the "lace rock" I think it is called) and some from the lanscape or quarry places. "River rock" as they term it is usually safe. As is slate that someone mentioned, and quartz. Limestone will work but only in tanks where you want basic hard water as it will slowly raise the hardness and pH by releasing calcium and magnesium. In livebearer or rift lake cichlid aquaria, this is ideal.
To ascertain if the rock is calcareous (limestone, dolomite, marble, lava are) the "acid" test is the No. 2 regent in the nitrate test kit; it will be labelled as an acid, and it is strong enough to test for calcium. But that is all it tests. There is no way to determine what, if anything, may be "absorbed" into the rock.
Vinegar can sometimes work for the acid test, but it is really not a strong acid--after all, we do consume it.