If you have a Tractor Supply local they carry crushed oyster shell and coral, 50 lbs. for $11.99
That is What I have in my Mozambique Tilapia tank. It is now very fine, but that does not stop them from sucking it up and spitting it out.
I gave up trying to keep it landscaped a long time ago.
The males are constantly digging nests in the substrate,
As noted, cheap and on-line don't really mix with shipping costs. I'll call around to some local landscaping shops for leads. Most of my aquarium rocks come from the farmer's discard field stone pile down by my families cottage.
if you have the ability to go get them, rock hounding is legal on national forest land. SWMBO and I decided that black rocks were going to be what we wanted for looks. Most rocks are not black but lava rocks are - at least some of them. Ordering the amount of black lava rocks I was going to need would have been over $1K. Instead, the kids and I went on a volcano adventure and got our black lava rocks from the Belknap Crater at the top of the Cascades in the Willamette national forest. This works for me because I live in Oregon which happens to have volcanoes in national forests.
Additional concern here: Lava rocks are sharp. The ones from the Belknap crater specifically are black basaltic lava rock - it's hard and it's heavy. In order to make this safe for the Mbuna, I have sandblasted each and every rock to clean and smooth them. Then I boiled. Then I fishless-cycled the rocks. The sandblasting alone has taken probably 15 hours.