Your best bet is surface collecting fields. Make sure you have permission of the landowner. If digging, you'll destroy any scientific value of the materials. In addition, if you hit graves, stop, put everything back, and cover it up. There are federal/state laws concerning desecration of graves (doesn't matter if they are prehistoric or modern). If collecting around a lake, be sure it is not on Federal property. It is against the law to collect materials from federal land without a permit. Same goes for state/federal parks.

Andrew