Large pebbles/ beach stones/cobbles (Wormbridge HR2)

I would like to make a bog filter for my pond so need some pebbles/cobbles for it.
I need 3-4” for the bottom layer, gradually getting smaller as they get towards the top. Basically any non porous stone eg granite, pebbles, flint, chunks of slate (not chippings) etc
Limestones are no good as they’ll alter the pH of
the water.
Thank you