Well, most of us in Germany order our parts from online because most car parts will be more expensive over here. Germany has Tuv which is a safety criteria for vehicles and equipment for vehicles. Everything that goes into a vehicle must be Tuv certified which makes everything more expensive. For instance, If you want a new set of BBS wheels they might cost 200 Euro each in Germany but you could probably cruise over to France or Italy and buy them for 100 Euro. Prices and munufacturer are of course hypothetical. There is a shop here in Germany (in Koln or Cologne) that has a pretty intense inventory. www.sandtler.de
they sell everything from a simple bolt right down to Sparco seats, turbos etc...I have noticed that the Sparco brand name is cheaper in Germany than it is in the US and they are Tuv certified so I imagine they are much cheaper in Italy where they are made. If you are going to be in Stuttgart, I imagine a drive over the Alps would run you about 4 hours into the Italian border. Just giving you an excuse to drive over Gotthardpass or any of the other fine Alpine roads.
As far as tuning goes, look into my posts and you'll see I bring a tuner to Germany because it is much cheaper to fly a guy over here to tune cars and perform instalation work than it is to get it done here. For instance, I fly Stephen over from IA Performance in Tuscon Arizona to tune. He does an awesome job, his shop has killer customer support and he can get you just about any kind of part you can think of at very good prices. Plus he loves coming to Germany once and a while to work on cars and get away from his shop for a couple days. Hell, wouldn't you? When I was initially looking for a tune for my GT30 setup, GP Tuning in Luxemberg (one of the more prestigious Subaru tuning shops on main laind Europe) quoted me $2200 for an Ecutek tune. I flew Stephen over to Germany for $525 and he tuned my car for $200. By flying a tuner over, I saved nearly $1500.
Handsone_Rob and me live about 2 hours away from each other but we still get together and head up to the Nurburgring or Sandtlers pretty often. Feel free to look up either one of us when you get to Germany. We are also on IWSTI under the same names and you can find us there.