Since no one answered the question because it was more important to scold you for not using Google, I'll answer your question -- NO, the OEM dark slide holders that appear on the late-model backs are not available separately. The Lindahl aftermarket and the Chinese "For Hasselblad" ones you see on eBay get mixed reviews. I find it easier to just slip the slide into a shirt pocket when shooting and only use it when I'm swapping backs. Of course in the four years since you posted your question, you're probably comfortable shooting with the Hasselblad, or have moved on or died by now, but if you've been relying on the slide as a shutter safety, be careful moving the camera around if you've already advanced the film and start leaving the slide out.
Originally Posted by Theo43