Luckily enough, these sanders are easily found at your local tool rental yard. Look in the Yellow Pages for "Tool Rental" or search the internet for "tool rental [your city]".
Drum SanderThis is the real workhorse of hardwood floor sanding. It's a massive piece of equipment, too heavy for most mere mortals to lift. Two people are required, and even then they should be very strong people.
The drum sander consists of the sander itself, a large dust bag, and a drum on which you attach large sheets of sanding paper. Be absolutely sure to buy the sanding paper at the tool rental yard, and you probably won't find it at your local Home Depot or Lowe's and certainly won't find it at a local hardware store. The rental yard employees will recommend at least three gradations of coarse, medium, and fine sanding paper. Be sure to overbuy sanding paper, as most rental yards will buy back unused paper (but confirm this first with them).
The drum sander rips off the bad spots, the high spots, stains, paint splotches--you name it--from your floor. It truly will take the floor down to bare wood. Unfortunately, if you're not careful, you can carve unsightly, permanent grooves in the wood. It's easy to strip off a quarter inch of wood without even noticing it.