Alot of us buy our blades from Harbor Freight. For a package of 2 the cost is Between $1.49 and $1.99 and they are called Carpet Replacement Blades item # 97642 They're website is www.HarborFreight.com just in case you don't have a harbor freight near you.
They fit 45 mm rotary cutters. And they are very sharp. :)