Well, you can always try Costco or Sam's club (or similar)... They usually have 16 or 32 blade packs that run about $2 per blade.
Some of the other folks on here buy them on E-Bay, but I haven't tried that out yet...
If you're worried about the costs, a 4-pack of Fusion blades runs about $13... one fusion blade typically lasts about a week, so you'll get a month out of a 4-pack. What's the normal haircut cost? (honestly, it's been so long, I don't remember)