Chris,
Be warned- W123's can rust severly. Mercedes uses an extensive layer of undercoating and it will "hide" lots of rust.
Sounds like the car will need a new swaybar and bushings plus hardware. I'd guess- $150 list from the dealership.
As for online catalogs- there are lots of companies and they use "worldpac" software. They are a big US wholesaler.
I use
www.thebenzbin.com frequently. Many people look at performance products catalogs.
If it needs a swaybar because the bushings are gone.. this car may need lots more $$$. Nothing more expensive than a cheap mercedes.
Michael