Product review (5 stars): Excellent, hard-to-find item!
This, apparently, is a very hard to find item.  I tried Home Depot, my local HVAC supply house and even an appliance store, and none of them ever heard of what I was describing.  However, this vent meets only the most specific of requirements, so be careful that it is exactly what you need.  The hole on the dryer and the hole in floor must match the dimensions of the vent both vertically and horizontally ... and, THIS IS VERY IMPORTANT, you must be able to reach behind the dryer from the side when it is pushed back to the wall or in even rarer cases, from the cellar below where the hole in the floor must be big enough to fit the entire vent pushed up and into place.  In other words, if the dryer is built-in, there is no way you'll get this vent attached.