I found few stores that sell larger rocks by searching landscaping supplies on Google maps.
Then clicked on one of the results and then click on the company website and viola
I did try that. Mostly small lava and round river rock here. Slate was an option, but heavy. I'll make the lava work, but for 60 lbs, $90 wasn't cheap. It looks much blacker when wet as well. I do notice a little bit of brown in places from sitting in the rain and sun. Not too concerned. Lava rock has some trace iron in it. Didn't bleach just rinsed thoroughly in a plastic kiddy pool!