Unless I've missed an undocumented renewable means of obtaining pebbles, both they and the rocky dirt needed to make them are not renewable, which means that obtaining large quantities of them for big builds is a bit difficult. I would like to see a renewable means of obtaining pebbles be added to the mod, much like how twigs are renewable by growing vanilla trees.
Here are some options you could take inspiration from:

• Breaking dirt has a chance of dropping one or more pebbles instead. Dirt is renewable through bartering for gravel to make coarse dirt or through growing azaleas on moss, so the pebbles would be, too. Many mods also add their own means of obtaining dirt renewably.
• Placing coarse dirt in a crafting grid would yield some pebbles. Because coarse dirt is renewable, the pebbles would be, too, and I have yet to see any mods or datapacks that have a recipe using only one coarse dirt.