MC Leaking?

I have a 68 Valiant with power brakes and I am wondering what common cause of fluid leaks? I see fluid is on the bottom of the MC when I look at it, But i can't see where the fluid is coming from.
If I have to, how hard is it to replace the MC on the car? I have included a picture. My son had the MC replaced by a shop about 7 years ago. The first one they got in the shop was the wrong part. So they had to send it back for a replacement. How hard are the MC's for these cars to find?
Can I get it from a place called rock auto?