I bought some plastic ones from the dollar store last year. It fits my Galaxy S6 and the LG I bought off my roommate for $20. It cost me a whole $5. This is the cheapest decent-looking one that I can find on Amazon:
https://www.amazon.com/GenBasic-Fold...rdboard+viewer
Surprisingly, the cardboard ones cost about $15.
I thought it was a fun gimmick, but my phone started overheating!