Atra blades are cheapest, and you can find "generic" Atra-clones dirt cheap. I get 'em 6 for $1 at Dollar General, that's the SAME PRICE as the disposables. Allows me to change the blade more often and save dough.
Remember, though, the adapter only lasts six months according to HeadBlade, and probably less the more you use it. You'll need to order a three pack at HeadBlade Dot Com.
Oddly, the "yellow" adapter for the Atra (Item #2005) is listed at $5.49, but I do not see the "black" Sensor adapter anywhere on the site. Weird.