I purchase my blades online from the following:
http://www.lpsharp.com/
you can also trade in your used blades, so don't toss them out, return them - they also ship free to you in the USA.
You may want to check out this information, I have no affiliation with this co., but very pleased receiving new OLFA blades 10 per pkg for $32.00, would normally cost over $100 in Canada.