What's wrong with your driveshaft? If your having a hard time putting them together, I would suggest cleaning the splines and then applying long lasting silicon grease to them.
Make sure that the splines or spline receiver on both driveshafts aren't bent or obstructed.
New ones are available. I searched using a 2003 M-B SLK 320 and came up with these:
LINK Driveshaft ebay LINK - Driveshaft MB Parts Giant
The cheapest thing to do would be to take them to a driveshaft repair shop. You can have them cleaned balanced and test fitted for a third of the price of a new one.
Post some pics of what's going on with yours so we can better understand what trouble you're having.