There exist 4 main porviders in Germany:
- Simyo (0,09 €/minute or text)
- WhatsAppSim (0,09€/ minute or text, free whatsapp)
- Congstar (0,09€/ minute or text, 0,24€/MB)
But you can get an overview here
If you go to a supermarket there is also the possibility to buy the phone card at the cash desk. Every supermarket offers its own tariff.