I just bought a 92 240. Car drives great once you get going, but getting going from a dead stop is a little slippery.

If you're really easy on the throttle, you'd never notice it. Anything beyond that and the transmission slips when you try to get the car moving. Once its got a little bit of momentum, the trans shifts perfectly.

Any advise? How easy is it to find used/rebuilt transmissions for these cars?

 

Try changing the trans fluid. If that doesn't work, used trannies are available all over the place--better than rebuilt.

 

I just checked the fluid. Dry as a bone.

Could it be just a leaky pan seal? What would be the possible causes of a dry transmission?

 

Leakage. Fill it up, see what happens.

Just because it doesn't register on the stick it doesn't mean it's empty, it's LOW. If it's been a long time may be there is no major leak. You should change it anyway, not FLUSH it, just change it.