They are available all over through spare part shops and of course as OEM through KL-varaosat.fi.
Motonet seems to have it as 10-4589 (37,90?) but they don't say what brand.
I would recommend Bosch only. I bought mine for the 300 through KL-varaosat and it cost 89,80?. The rotor cost 34,90?. But these prices are for the 300, YMMV.
It's possible some place like AD-varaosat or Osaset or similar might have Bosch for cheaper.
By the way, you could try sanding off any carbon/oxidation in the contacts of the old distributor cap, to confirm that it is causing the rough running. It won't be a longlasting fix but it will give you peace of mind if it clearly shows that the cap is the culprit.
That being said, sounds like you clearly have leaky injectors as well. Do you (or a friend) have a setup for testing injectors? I still have my old ones from the 300 and we have the same part # injectors. I could borrow my injectors so you can test if maybe some of those are in working condition and you could see if it improves your engine feel.
The 6x Bosch injectors set cost 177,75? on eBay including shipping. Now there seems to be a cheaper seller at 152? incl. shipping:
http://www.ebay.de/itm/6-x-BOSCH-Ei...l-Mercedes-W124-W201-0437502047-/121991568557