I buy the Olfa blades at Wal-Mart on the rare occasion they go on sale.
Blades & needles do add up, but I personally had too many injuries trying to work with dull ones (same thing with knives).
The trick with sharpeners is that they must re-sharpen the blade at the same angle at which it was originally sharpened. Otherwise, it's not going to do much good. I haven't been able to find that information out for rotary cutters (it's available for knives through manufacturers or sometimes even with literature in the box) but if you could match up the angle of a particular brand of blades with a particular sharpener, it would work.