Well, we aren't part of Berlin here, so no wonder you don't find anything. Brandenburg (not -berg) is the State that surrounds Berlin. Also, Brandenburg an der Havel (B'Burg-upon-Havel) is a midsized (pop, 71,000) city within the State of Brandenburg. Read all about it
here.
If it's a Hertha hoodie you want, I suggest you select Spandau as your burrough, because
1 - It's the Burrough Arneb grew up in
2 - It was the largest Berlin burrough in the British Zone when Arnebburg was under Allied occupation (that is, until 3 Oct., 1990)
3 - It was the place where you Brits and the other three kept that Hess arsehole prisoner until he did what you should have done 40 years earlier.
4 - Spandau is five years olde than Berlin (first document containing the name in 1232, Berlin, 1237 AD)
5 - Spandau ROCKS, not only with the Ballet. Seriously, it does. We have a citadel, we the church where Brandenburg converted to Protestantism, we have the most beautiful Havel riverside, we have the BMW motorcycle plant, we're fucking AWESOME.
If it's Potsdam you're after, they don't sell hoodies but you can get the
T-shirt.
Since I live in Babelsberg (what could have become the German Cinecitta, if not Hollywood, without the fucking Nazi scum), you might have a look at SV Babelsberg 03, our local 4th-Leaguer. They are a staunchly independent left-wing club, with a proud pedigree of opposition during GDR times. Theey are clearing out their
shop, so great gear competitively priced.