Small, smooth stones will help toughen and exfoliate the sole without bruising it if your horse is thin soled. Helps to build it up. For the pea gravel, if you have a less than ideal soil type, could always make a base with large stones and hardware cloth underneath, then top with gradually smaller stones, but can be cost prohibitive.Crushed limestone seems to at least provide a dry area that packs down, and helps in preventing thrush. If you live in a naturally rocky area, just don't pick them out of your pasture.
I currently live on a super sandy soil, and pea gravel just disappears in it. I put every rock I could find here, around the water trough, and as the horses pack them down, I add more. We are building up a 12 foot radius around the waterer, and will put landscape timbers down to retain the rocks (the area around it has been badly eroded, so we are filling it back in this way) and top with pea gravel.
Even if you can't put rocks everywhere, just put them in one key spot they have to go.