Best bet would be to get the parts from a Euro junk yard. I have a W126 with a manual trany but it is gray market. I have heard that a lot of the cars were manual in Europe. I believe some sl's came over to the US with manual transmissions so that may be a source here in the states. If you find a donor car it should not be hard to do. There would not be any need for aftermarket pedal boxes as the chassis comes with a clutch on some models with them. All you need is a clutch master cylinder and possibly a smaller brake pedal. The master cylinder is located inside the car and gets fluid from the brake reservoir. You may even be able to use the 6 cylinder transmission. The shift rods may be hard to convert if they are from a different chassis, but even that is not that hard.
My wife would love for me to convert the car to automatic, but I say no way! It is kind of strange driving a big sedan with a manual but I like it!
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater
got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)