The components of soil mix are put together to hold water and encourage root growth. Pebbles do nothing to hold water. You could use a very thin layer to be decorative on the top of the soil, but that is not a usual bonsai thing.
Last time I looked the lava rocks were pretty big, 5 cm or so, about 10 times what you get with bonsai grade lava rock.
Find a local club and see what they do. They may buy in bulk for club members.
