Look for giant puffballs on the ground in well-fertilized fields or pastures where the underlying fungus has plenty of underground manure to decompose.
Giant puffballs also grow on the sloping areas of bare earth in urban areas, places where people tend to discard litter. Here the puffballs hide, disguised as Styrofoam.
I once led a tour that first met at the base of a wooded hillside in Queens, NY. We piled into cars to carpool to the park's interior. I got in last, to make sure no one was left behind. Just as I was about to close the door, I noticed that the entire hillside where we had been sitting for 20 minutes was covered with giant puffballs, which I announced at the top of my voice.
Everyone raced to the hillside, and there were enough enormous giant puffballs for everyone to get one, except for me. But after the tour, I raced home on my bicycle, dumped all my gear, and continued in the opposite direction to a park where I'd found a large number of rotting giant puffballs the year before. This time, in failing light, I got them all, filling up shopping bags, backpack, carrier rack, and panniers.