Amazon.com: for ATMEL 51 AVR USB ISP ASP Microcontroller Programmer Downloader with Cable + 10Pin to 6Pin Adapter Board for Ender 3 or Ender 3 Pro Geekstory : Electronics
AVR Development Kit - Tutorials
AVR ISP Pogo Adapter kit from Geppetto Electronics on Tindie
Standalone AVR ISP Programmer Shield Kit - includes blank chip!
Amazon.com: for ATMEL 51 AVR USB ISP ASP Microcontroller Programmer Downloader with Cable + 10Pin to 6Pin Adapter Board for Ender 3 or Ender 3 Pro Geekstory : Electronics