I got online to Harbor Freight and purchased their carpet cutting blades. At the time I got them for 2 for $1.99. I ordered 20 blades and shared with my Mother. They do have stores and they can be purchased online. Different, somewhat. They have a round hole in the center of the blade. The Other name brand blades have a notched hole. They still work well. Each package comes with two blades. I was concerned if they were sharp enough for carpet would they cut up the mat and I can say they don't. Do they last longer? Well I think they are about the same as the name brand. When I get closer to using all of the ones I have I will order again.
Here is the website for the company. http://www.harborfreight.com/ In the search box type carpet blades. It will show several styles but only one round blade.