Where can I purchase a suitable smartphone for the smart3Dscan app?

You can still get older models of the iPhone on these platforms. We recommend the iPhone 10 (X) to 12 (excluding the SE models) and the iPhone 12 Pro:

• https://www.refurbed.de/c/iphones/?n17=1.2-12&sc=true
• https://www.backmarket.de/de-de/search?q=iphone%20x