Yes, they are available. I have a '71 250C with Webers. JAM Engineering (John Moulton) is on the West Coast, advertises in Star and gives a discount to MBCA members. Dont' have his e-mail or phone number here but I can get it to you if you like.
Holly carb's from Ford Pinto's will also work. I have never seen them myself. I understand that those Holly's were actually made by Weber - so that is probably why they can be used.
If you go w/Webers (they really are neat) make sure you get them jetted correctly or you will have a terrible time (don't ask me how I know that).
Here are the Weber jet settings you should have (both carbs are jetted the same):
Mains: Pri: 127 Sec: 130
Air Corr: Pri: 170 Sec: 175
Idle Jets: 50 and 50
No mods to the intake are required. If you get the complete kit, you will be able to use your stock air-cleaner. You can also get K&N Units (I did) that bolt directly to the Webers. Permanent filters - you just clean and re-oil (charge) them. Look and sound great.
Let me know if you need any other info.
Dan Taylor/Tulsa, OK MBCA '71 250C/'81 300TD