Yeah retail prices. And yes you can buy them in a store like home depot or order them on the internet but shipping is prohibitively expensive for small amounts. Cheapest source are local firms that install PV professionally and sell via kleinanzeigen.de as a side business or to get rid of excess stock.

Thanks for the clarification, also on the use of the word diffusion. In social sciences it is common though, there is even a book titled “Diffusion of Innovations”.