Random construction question.

[BengalHawk62]

I recently had my siding replaced on my house last week. Well over the weekend I was outside walking around the house and noticed hundreds of nails laying in the rocks and grass that no one took the time to pick up. Now I'm worried about getting a nail or 12 of them stuck in my mowers tires. Does anyone know a good way of picking up nails and other loose metals laying out in the yard? Is there a big magnet I can purchase for this?