Generally speaking, manual transmissions are hard to come by in Mercedes cars here in the U.S. Here's a brief rundown of post-1985 models that I know had manual transmissions available here:
W201: 190E models with the 4-cylinder 2.3L 8V, 2.3L 16V, and 2.6L 6-cylinder engines were all available with manual trans. over the years. Not common but they're out there, the 190E would be the easiest model to find with manual trans.
W124: 300E models in 1986 and 1988 were available with manual trans but are very rare and hard to come by.
R129: 300SL models in 1990 and/or 1991 could be ordered with manual trans but again seem to be rare.