The people who lived here before us dumped a whole bunch of nails, and glass, and metal at the back of the property. We were unaware of this until we started tearing out trees to make way for a clearer horse pasture. Now the debris has been somewhat scattered around a certain area of the field. We can't bring our horses home until we get this taken care of. My question is does anybody know of a place where we can rent a fairly large and powerful magnet? Or a place we could buy one? What kind of magnet would work best in this situation? Please, any help would be greatly appreciated.

Do you have a Harbor Freight near you? They have a magnetic pickup that has wheels. It is a bar about 10" long. Check HD or Lowe's, they use them in the hardware dept. to pick up nails and might rent them. The glass is obviously non magnetic and you'll have to hand pick it.

How about getting a metal detector.

If your having real issues like finding debris fields, you can have the area mapped with GPR (Ground penetrating Radar).

A magnet: http://scientificsonline.com/product.asp?pn=3071135