The serial number on a "Red Nine" is where it is on all those pistols, on the sloped surface on the left upper part of the receiver.
I can't be certain without some better pictures, but I am pretty sure that is not a real gun at all. It looks like one of those dummies made up for collectors in areas where real guns are illegal. That would explain both the plugged barrel and the lack of serial numbers, and also the reason that you got it so cheap. (If it is a dummy, you didn't do too badly, those good quality dummies run as much as $100.)
I STRONGLY recommend you take that gun to a gunsmith or someone knowledgable before spending any money on it or even attempting to fire it with ANY ammunition.
Edited later to add: My first suspicion was focused on the grips; too new looking, the red color too bright. But I have been comparing that gun (the parts pictured) with a genuine Red 9 and still think the above assessment is correct. But it is good, darned good. I had to look at some real details to confirm (in my mind) what I suspected, like the thumbpiece on the safety being a separate part where on the real gun it is part of the safety and the grooving is different. And too many serrations on the bolt. I admit I am waiting to see if what the OP says. If it is a dummy, the OP need not feel bad; I think that "gun" would fool just about anyone.
Last edited by James K; May 10, 2012 at 09:02 PM.