I took a large backpack out into the woods. Twice. Came out with a hundred odd lbs of very nice rock, hand picked, uniform in color. I also found a great collection of nicely rounded streambeddy rocks in a drainage coming off a cornfield. Small to quite sizable, and again they all sort of match each other. (WARNING: this rock thing can go to far -- I now need to return a hundred lbs or so back to the wild).
I personally can't see the point in paying perfectly good money for a rock I can pick up off the ground. I'm going to assume ND has just as much ice age rubble strewn all over everything as NY. Terminal Moraine, baby. I would stay out of the garden at McDonalds, and anywhere else you might see a guy watering with a tank of blue chemicals attached to a hose. My farmer fertilizes with manure (dairy farm) and doesn't spray anything. Wilderness=good.
As far as the iron goes: I added laterite to my tank and my next one will most likely have a flourite substrate. These are used by a lot of the folks out here (esp. the plant folk) and are used specifically to introduce iron into the tank (for the benefit of the plants). I doubt your rocks are going to introduce more iron than that. There are iron test kits if you're still in doubt, it never hurts to let some test rocks sit in a bucket for a week and then test KH, GH, pH, and iron. If you're not happy with what it does to the bucket, keep it out of your tank.
And if anyone needs some nice rocks…