IR has good rachet backbands in two styles - one that has the rachets behind the backband, and one that has rachets up near the thighbraces on the inner wall of the cockpit rim (forward adjust). IR doesn't recommend the later for kayaks that didn't originally come with a forward adjust rachet - but I've done it and seen it done before. The backband with rachets behind it can be a real pain to adjust, though better than a non adustable backband.
Another option is the Jackson Kayak rope/cleat adjustment for the backband. You can retro fit it to use an existing backband and only buy the rope/cleat, or you can buy the kit that includes a new backband. I bought a rope/cleat kit this wekeend so I can retrofit my kid's WaveSport Fuse with a forward adjust backband system.