Which transmission do I need?

I have a 1999 Mercury Sable that needs a new transmission. According to Ford the transmission is a C/DX - 4 SPD Auto TR NAAO AX4N/4F50N. However this can't be found online at any of the transmission sellers. What I do find is A.T. (6-183, 3.0L) OHV (VIN U) AX4N (Code X) Column shift or A.T. (6-183, 3.0L) OHV (VIN U) AXODE and AX4S (code F or L) column shift. How do I tell which of these is the one I actually need?

The answer you seek is in the question. You said according to Ford the transmission is C/DX - 4 SPD Auto TR NAAO AX4N/4F50N, the last bit is actually the name of the transmission AX4N which was renamed 4F50N in 2001. If you want to verify this check if you have 19 bolts on the fluid pan or an X on the data plate. If you count 17 bolts or an L on the transmission data plate then Ford was wrong and you have an AX4S.