US patent number 6840836B1 appears to have lapsed. How do I acquire this patent?
You can contact the rights-holder by looking through the USPTO assignment database to see who is the last recorded owner. You can contact the last rights holder and see if they are indeed the current owner (there is no obligation to record assignments, although it is good practice). http://assignment.uspto.gov/ Once you have contacted the rights-holder, you can negotiate a deal.
It is possible to assign expired patents, however your rights are severely limited so make sure you know why you are buying a lapsed patent. You are unlikely to be able to revive the patent yourself, and you need to do serious diligence if you ask the rights-holder to revive to make sure the patent isn't invalid after the revival.