Is it possible to use a scout upper on a standard, full size lower?  I have a standard M1A with the synthetic stock and wanted to switch between the two.  One for long range work and the other for short range work with an Aimpoint.  

If it is possible, does anyone just sell the scout upper?

Scout upper? You are taliking about an M1A, right? If you mean will the stocks inter change, yes they will. They are the same. I may be missing something here but it sounds like you believe that the M1A has a barreled upper receiver group and a lower receiver group that houses the fire control parts and magazine. Like the AR-15. Well, and please excuse me if I miss understand what you're asking, the M1A has only the barreled receiver. Separating the barrel from the receiver is not as simple as swapping uppers on an AR-15. You could put a Scout mount and Scout hand guard on your standard M1A.

Your right.  My mistake.  I mean can you use the same trigger group and stock for the M1A scout and a standard M1A.  I would like to interchange between the two.  You know.  Sometimes you feel like a nut.  Sometimes you don't.  

Any suggestions as to who makes a good M1A receiver to build my scout on.  Is it worth getting a single or double lugged receiver?

Quoted: Your right.  My mistake.  I mean can you use the same trigger group and stock for the M1A scout and a standard M1A.  I would like to interchange between the two.  You know.  Sometimes you feel like a nut.  Sometimes you don't.   Any suggestions as to who makes a good M1A receiver to build my scout on.  Is it worth getting a single or double lugged receiver?

Im not sure what you are trying to accomplish here. You want to have a stock and trigger group that you will use on 2 different rifles?  Those two parts are cheap enough that you can purchase them and have 2 complete rifles.  
If you want a reciever go and find a smith arms.  Im not sure of the status or quality of the New MKS recievers so i cannot comment there.  One of the other companies who used to make forged recievers is making them again (forget the name, maybe enterprise?) for around $500 a reciever.

Quoted: Any suggestions as to who makes a good M1A receiver to build my scout on.  Is it worth getting a single or double lugged receiver?

Look for a Poly Tech or a Norinco M-14S receiver. They are the best semi auto receiver available and they are inexpensive.