Hi Stilv , thats one of the RA or Dec lock bolts ( levers ) , I dont know where you would get one new , but I am sure some one here would have one .
For a quick fix I would go to Bunnings and buy the smallest " Vice grip " plyers I can find , ( I have seen them 50mm long ) and lightly grip the bit thats on the mount , with screw fitted back in and use the vice grips as a handle ,,
that way the mount is still useable untill you find a replacement lever , try Bintel .
Brian.