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Silly as it may sound, try your Mercedes dealer for some things. I approached mine over 15 years ago and just told the guy behind the counter that I could buy parts from him or I could buy them online, and I would rather buy them from him.
Since that day I have been paying jobber prices at the dealership.
While they're not always the cheapest, there are some things like suspension, brakes and steering bits that you don't want to buy aftermarket. Consider that while they might be expensive, you're probably replacing an original part with one that will last as long. Do the math and it makes sense.
Soft parts like filters can be sourced any number of places, since there are several OEM manufacturers, such as Hengst, Mann and Mahle. I like to use these
guys for Mann filters. They don't always stock what I need, but their prices are really good, especially if you buy in bulk. If you plan ahead you can get filters from them for far less than even places like AutohausAZ and others.
Understand that the really, really important thing when it comes to aftermarket parts is to have a good parts guy who knows the business and can give you choices, as well as steer you away from the crap that's out there. One name you want to avoid like the plague is URO.
Anyway, I have a long time parts guy who really knows his stuff when it comes to suppliers, and won't hesitate to tell you to go elsewhere if it's a better deal for the right part. His name is Gary, aka Dr. Fatty, of BuyEuroParts.com - Wholesale European Parts - Mercedes / BMW / Volvo / Saab / Porsche and more!
Gary has been in the business for many, many years, and has a very good understanding of what suppliers are good and who aren't. This is especially important these days with the rash of cheaply made knock off parts that come out of Asia. For example, go look up a part for your car and you may see a very wide range of prices for that part. At the bottom will probably be URO, then in the middle the OEM suppliers will cluster, then at the top will probably be OE (Mercedes). When the difference is 10-20%, it rarely makes sense to go aftermarket in my humble opinion.
Some parts for your car might be hard to find, which will point you to the MB Classic Center. Tom and the rest of the guys there are very good, but parts from the Classic Center are often very expensive. Amazingly, they are often available through the regular dealer distribution as well. if you have a good relationship with your dealer this can save you some serious coin for those near "unobtanium" parts.
Ask a lot of questions. Do your due diligence before taking on a repair. These cars aren't that difficult to work on, and with the age of yours many, many of us have gone before you, so there is a lot of stored knowledge out there to tap into here and on other forums.
The 108 is a really cool car, and one of the best looking (after the W111/112 in my opinion) Mercedes ever built. You're ahead of the game in a lot of respects because you have a fuel injected model. The carbureted models were as good, but some people find them difficult to work on. That's another story....