I recently picked up the rotary blades at Harbor Freight and couldn't believe the price -- a fraction of the 'official' Olfa blades. The blades are not exactly the same size as the ones for my 60 mm cutter, but they works
just fine. I find them at least as sharp, if not a little sharper. While I'd probably prefer the ones made specifically for the Olfa cutter, the price difference makes the Harbor Freight ones definitely worth it!
The biggest upside is that I don't hesitate to change blades the way I did when they were several $$ apiece!