Replacement screws can be bought on Amazon - search 3mm laptop computer screw kit
I have a mid-2009 MBP. Removing the 3mm screws was nearly impossible. I read that Apple uses loctite glue to secure the screws. Someone recommended touching the hot tip of a glue gun (minus the glue, of course) to the screw head. Doing so heats the screw and loosens the loctite. It worked well for the first one, I rushed the second and striped it so badly that I had to use a drill to bust up the screw head. I didn’t have a bit small enough to breakup the entire the screw, but I was able to pulverize the head so that the mounting surface was flat, which enabled me to install the new cable. At the moment, my cable is held in on that side by just the one screw and the weight of the hard drive. I’d like to replace the missing 3mm screw but I’d have to drill out the rest of the broken screw to have hole to put it in. Not exactly looking forward to that….perhaps if I just use some electrical tape to secure it down? On the upside, the new cable is working and my mac is up and running again.