I went to KRC Rock in Poway it's a landscape rock place. I asked if I can get some broken pieces and they said go ahead and I did not even have to pay for them as they can not sell broken pieces. They have a nice selection from Lava rock to Flat Slate. I'd ask before picking them. They have heavy duty plastic bags to put them.
The KRC Rock in Poway is 12249 Oak Knoll Rd. Poway (858) 748-3953
They have another location in Lakeside also that you can look up. You might want to check out Landscape rock places close to you, Better then the local fish stores for rocks and you might get them for free or really cheap.