Just had a look on Google Map, your based in the middle of no-where land where everything is just that little bit to far and anything local really is local ... nothing to buy unless you are willing to travel some distances This keeps coming up and looks to be your best suggestion, if you can't speak/read German its still easy to navigate
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Your best places seems to be the main towns, Sarrbrucken and Mannheim are really the nearest, but if your willing to travel Franfurt or even Luxemborg, so you needed a car/bike to get about I was based locally with the British Army when in Germany, we were in Dortmund and the shopping centre was only 15 minutes by car so got lucky, but the next posting was about an hour to get anywhere that had a big town like Hannover or Hamburg
What about Amazon Germany, you'll probably get the cheapest from that and postage shouldn't be that much. Assuming they will post to military addresses some companies get awkward with things like that I live in UK, and bought two Nikon lenses off Amazon UK, with free postage, and arrived in original Nikon boxes with all paperwork and were about the cheapest I could buy for new original Nikon lenses, which were the 18-55mm and 55-200mm AF-S lenses
Buy UV filters for your lenses, not only will it filter out the UV light it will also protect your lens from dust and scratches and if you do happen to drop it, you might get lucky and only smash the UV lens. Much cheaper to replace than a new lens
Also the generic battery grip available for the D3100 is recommended by a few as it adds stability, can still be attached to a tripod and has the added advantage of taking non-Nikon batteries, which are about 75% cheaper than official branded Nikon batteries and also means you can take more pictures before having to charge them, or buy one original Nikon battery and carry that with you when out. But the D3100 can apparently take upto about 400 shots with one battery charged. I have taken more than that in a full day when in Egypt on holiday/sightseeing so it is easily possible
I also bought a small camera bag/back pack so you know everything is all in one place and makes it easier to carry your camera gear about
Have a look at external flash guns if you use flash more than a couple of times. I'm also still not sure weather to go for the Nikon SB-400 flash or buy a YN-565 flash, but the Nikon SB-400 is very compact and I don't use flash that much to justify a much larger external flash as I'm also thinking about portability when out and about Now I've just found the Nissin Di-466 which is compact but almost 40% cheaper than the Nikon SB-400, so that adds even more choice between the Nikon SB-400 or Nissin Di 466 or YN-565