Like Tracie, I too went to my local landscaping place. They usually only do yards, half yards, and the like but I told them I only needed a small amount for a DIY project and they let me take out a bag and shovel. I filled a whole paper grocery bag for $5. Needless to say I have a TON of leftover rock that I can build with =]
What you see in the picture is about 1/10th of the total amount of rock I got. Maybe less than that. However, when you do this be mindful that these places sell the rock to be used outdoors so they are VERY VERY dirty as they have no reason to clean them.
I washed the rock you see there for about an hour, boiled them, and washed them with vinegar and I still notice some dust/dirt particles on the white paper towel underneath.
The rock that I got is called SlateScape at the local store. It's also called Slate Chippings, Slate Aggregate, or Crushed Slate. I couldn't find any at our local Lowes or Home Depot, but a lot of members on AA have gotten their slate from those types of places. Most of what I found though was slate flooring which can have additives.
EDIT: Tomorrow (Technically later today after I sleep and get out of work) I should have a picture of the 75% completed structure.