Plant substrate or just substrate?
Cascade Rock in Sac has some great dolomite substrate, river sand, and some nice gravels.
I'm told that Tractor Supply in Woodland has Black Diamond blasting sand, which is black and looks nice. Lots of good nutrients too, but a low CEC.
You can also use Schultz Pond Soil, which ACE in davis has (if you ask). Lightweight (too lightweight), but cheaper than most aquarium options.
You could also try the John Deere stores for a clay-based substrate. Turface, maybe?
EDIT: Oh, and Delta mud. Ask Tom Barr.