I'm not familiar with the term 'redrock' (unless its that nasty red cinders), but if it is, and if you are looking for larger sizes of pumice aka 'featherrock', there is a local distributor in Redding, north of you.
Pumice, lava, cinders--they are all volcanic in origin-just different textures and uses.
Axner Excavating 530-222-0539
I have purchased huge bags of vermiculite at Home Depot, as well perlite.

Just a note... I'm still learning how to help with resources. Forgive me if I'm not posting correctly. And please, let me know how to be better if this is wrong. Green Planet Naturals has garden pumice in bags ready for shipping.

Here is a link that might be useful: Green Planet Naturals