For my river tank set up I went to a well stocked landscape supply, rock by the ton, but will do by the pound, and although I still spent too much money, it wasn't because it was expensive but because I like rocks too much, lol!
I was able to get Mexico river stone. Beach pebbles and tiny stones, they called sand but were around ~3 mm long looked like teeny river stones.
Here is that thread where I showed some of the stones I bought. And examples of a bunch of them.
FW & SW
Favorite tank right now - 72G Bowfront Wadai Tetra, Cardinal Tetras, Ghost Glass Cat Kryptopterus bicirrhis and Clown Loach Cory haven
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article