I have the Lowepro SlingShot 200AW for my K-x kit. It will accomodate my K-x with the 55-300mm lens mounted, as well as room for the 18-55mm, a 50mm 1.7 prime, an AF280T flash, and one spare space still. The top zipper compartment contains a small pair of glasses, a tabletop tripod, pen, and a remote control; still room to spare. The front compartment has a couple of extra sets of batteries and is
just the right size to hold my Operating Manual.
I'm really happy with the bag. It's comfortable to tote and you can swing it around, unzip, and have your camera out, turned on and ready to shoot in a few seconds!
I don't see this particular model on B&H anymore, but I'm sure they'd have an equivalent.