Most Ace Hardware stores have large selections of mm machine screws with hex heads (either button, taper or standard). They are alot cheaper than buying from the dealers. I cannot think of any phillips head screw on the bike that needs a special hardness. One of the first things I do is replace all the phillips with hex head from Ace. Also anit sieze compound may help, but you need to make sure the manual does not call for loctite and that you get the specified torque correct. This doesnt help get the screw out but it may help solve the problem in the future.