Good tackshops are getting rare. Mostly you find superstore type chains such as Kraemer or Loesdau who have branches throughout the country. Thüringen (the province Jena is situated in) is tackshopping-wise a bit of flyovercountry (sorry to all who live there)
Here is a map to the Krämer superstores http://www.kraemer-pferdesport.de/tpl_megastores.htm?websale8=kraemer-pferdesport&tpl=tpl_megastores.htm
And this one is from Loesdau these are the biggest chains in Germany and carry pretty much most of the German brands for day-to-day tack, apparel and supplies.
http://www.loesdau.de/Pferdesporthaeuser.htm?websale7=loesdau&tpl=s-f-filialen.html
If you get anywhere into Lower Saxony or Westfalia that’s where the best tackshops are (as in actual shops with an owner who knows what they’re doing and often run in the family for generations).
There are a few smaller chains as well that are very good and better assorted than the mainstream warehouse type stores above. As long as you’re not looking for anything special you may find what you’re looking for. Better bargain potential is with the smaller owner-run shops who often put significant discounts on last year’s collection e.g. Eskadron brings out a new kinky color every year and then last years kinky colors go on sale lol
These ones are well assorted but out North. They used to have a branch not too far from me but it was sold.
http://www.die-reitsport-preisbox.de/
This is a shop I often go to but it would take you a few hours from Jena:
http://www.reitsport-exclusiv.de/f.php/shop/reitsportexclusiv/f/627/lang/x/kw/Sonderangebote-Restposten/
If you have a chance you also want to visit Luhmühlen where you have good tackstores everywhere carrying quality stuff. Most of the leather sold by the big stores is crap I would stick to the better brands for leatherware. The brandname stuff you can obviously buy anywhere.
If your schedule doesn’t allow for touring the countryside here’s what I found locally:
http://www.dressurshop.de/
https://plus.google.com/109426808586783071708/about?hl=de
I can’t tell you if they’re any good though. The second one also does carglass repairs so might be not a full sized operation. Thüringen and Sachsen have very different infrastructure from elsewhere in Germany.
If you happen to come through the B61 at some point, drop in with us for a coffee I can tell you everything about the tackshops in our neck of the woods 
The big warehouses offer online-service so you could have shipped stuff to your resident address. Watch import-restrictions/customs if you buy tack you might want to use it over here for a few times so it is more obvious you’re not bringing it for merchandise. US customs tend to be picky but I guess less so with their US residents hehe. I know of several riders who got in trouble because they let immigration know they were bringing riding boots to be able to give clinics, they didn’t like that info…
Enjoy your stay!
