Rollerblade Crossfire Inline Skate Rocker Axles are intended for Rollerblade rockerable frames like those found on Rollerblades Crossfire Skates. RB offers several rockerable frames now which is cool. All can be found at Inline Warehouse when available (see other items to consider below). They are 8mm axles, so they may work with other brands as well.
 
They come in a pack of 4 as there are just two rockerable axles needed per frame.