All the hardscape I own I bought over the years from LFS... for me it's an issue of "art" and laziness... if it looks good I buy it, out in the wild I'm too lazy to carry it home.
Besides, I "think" there are ordinances against taking objects from parks, beaches, etc.
The few backyard rocks I collect I spray with 10% bleach(baylisascaris and samonella)and scrub with dish soap, lots of rinsing in between. Put in the aquarium(new bare) and test the pH, then decide what ecosystem I want to setup.