The only place that I have ever seen master cylinder covers available is on the rotating displays at places like NAPA, Carquest, Autozone, Advance, etc. They are fairly universal across the board on GM cars of the same vintage, however, the bore and stroke of the cylinder itself might be different. If the top cover is rusted through, then I would be very suspect of the cylinder itself. Check to see if the fluid is clear. If it is dark in color, or shows a milky look, the fluid should be changed in the system, and the master cylinder checked for contamination damage.