Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Go to landscape stores, masonry places, rock yards and so on.
Most of the rocks sold there, in bulk or in bags are aquarium safe.
Lake Victoria water is harder and a bit more alkaline than many of the tanks people keep plants in, so even a rock that will add minerals like calcium or magnesium to the water will not be a problem.
If you can get a few broken chunks, samples that you can bring home and test that would be great. Put these samples in separate jars of water and test at the beginning and every few days for pH, GH, KH and TDS. Minor changes in these should not be a problem for hard water fish, but if they change a lot that might be too much.
Anubias should not be planted in the substrate. The roots are better at anchoring to rock or driftwood, not into the substrate so much.
I would get the rock you want, then use the gel type of superglue to attach the Anubias.