Here is the information to buy them at Harbor freight. "package of two carpet cutter replacement blades. Item #97642.
I first purchased these at harbor freight when it was first mentioned on this site. Of course the store was out so I ordered them online. I bought 60 packages for me and my friends. (I plan on quilting for a long time :) )
When I ordered these online it took over a month for them to arrive because they were on back order. (All the quilters must have found out about them.) I guess another reason I ordered so many is because I thought maybe harbor freight would figure out how much they could charge for these and I wanted a stash before that happened.
I have not noticed any difference with these blades compared to any other brand. One person did mention that they made more marks on their mats but I have not noticed this.
Oh my, after typing this I think I should order more. I could always will them to my daughter if I didn't use them all. :)