I agree that hiking is one source. But remember they are heavy and will be a pain to carry out.
Drive around in lake communities in the Highlands region (Fayson Lakes, Kinnelon, Montville, Ringwood..) and look at the rock walls around the homes of people. Try knocking on the door and explain why you would like to take a rock from their wall. Maybe even bring a few photo's of planted tanks to show them what you are trying to do. This only works if they haven't purchased the rocks to make the wall. You can also try bringing some rocks and just trade for it.
I have found many flat rocks in my own yard while digging up my garden. I've saved them over the years. I have rocks that have visible iron staining in them and use them in my tanks without any problem. I figure the plants may be able to use that iron. Lots of iron in the Highlands. I took some rocks from the blasted area near the Walmart in Riverdale on Rt 23 N. They really did have a huge amount of iron and I didn't end up using them.
Here are some of them:
My 65 gallon Tanganyikan tank thread is in great need up updating, but you can see some of the rocks I've dug up and collected in Northern NJ. This tank just went 6 days without power and I am left with only one Tanganicodus irsacae and a pair of Julidochromis regani and one Synodontis upside down cat. I removed about half of the rocks to look for the dead fish and have not returned them yet. The tank is pearling like it is CO2 enriched. I did change a PC light that burned out with the power failure.
65 Gallon Tanganyikan Tank with Vallisneria/Algae
Don't worry about the use of limestone rocks in your tank. Your water changes will prevent a huge build up of Ca hardness. NJ limestone is mostly dolomitic limestone by the way, especially the limestone found in the Ridge and Valley Provence cut by Rt 80 west of Allamuchy. You will drive by some really nice cuts and see some good rocks-but how to get them. Exit and see if you can find rock walls again and ask.