One of the easier tractors to find parts for. Most AGCO dealers have parts or can order them, the secondary aftermarket parts are readily available and the there is a good salvage maket for parts. What tractor are you looking at?
There are a few of the foreign made 70's and 80's models that are impossible, the parts are made from unobtainium.
The WC/WD/WD45 and the D series are all good. The 7000 series and the 8000 series are still in in active service and most AGCO dealers can get you parts.
As was previously stated, Norm Meinert is the source for the older models. Doc Smith in Lacon, IL, Bill Deppe in Moqueta, IA, Sandy Lake Implement, Dean AG in MO and a whole host of others.