LP Sharp Company (lpsharp.com) takes trade-ins. Send your old blades, get new ones for considerably less. Haven't done it yet, but I'm saving my used blades because there is a 5-blade minimum. Also, I've heard Harbor Freight has blades for carpet cutters that fit the rotary cutters and, again, are less expensive. Alas, haven't tried that either. Seems that when I need a new blade, I need it NOW!
FYI, I bought one of the electric sharpeners and had no luck with it at all.