Habeneros aren't common there, but in general for cheap chilis, try the Turkish grocery stores (EuroGida or similar). Basically for any Mexican / Latin-American stuff, they're great for sourcing ingredients: chilis, dry beans, corn meal, flour tortillas (durum) and will be a lot cheaper than specialty shops. Chilis are also easy to grow (I'm growing 4 kinds on my balcony in Berlin), and one habanero plant goes a long way.
Not in DE myself but scotch bonnet chillies are more easily found in the UK due to the Carribbean/West African connection. You may have a better chance of finding them instead and I'm told they are quite similar.
Usually you can find them at super markets (Rewe, Edeka) sometimes Discounter have it as promotional item (Aldi).
I recommend growing yourself just use seeds from the bought fruits. They are rather easy to grow.
I can also send you seeds, if you like.