This was the cheapest source I was able to find. It cost $3.50 each.
Now I'm planning to design a development board, what fits into my ESP programming "system":
http://pakahuszar.blogspot.hu/2017/02/project-20171-esp8266-programmer-1.html
http://pakahuszar.blogspot.hu/2017/03/project-20171-esp8266-programmer-2.html
http://pakahuszar.blogspot.hu/2017/03/project-20171-esp8266-programmer-3.html
http://pakahuszar.blogspot.hu/2017/03/project-20172-esp8266-breakout-board.html
Collected some information for the hardware design.
The ESP-WROOM-32 datasheet:
https://www.espressif.com/sites/default/files/documentation/esp_wroom_32_datasheet_en.pdf
Schematic of the Espressif development boards:
https://www.esp32.com/viewtopic.php?t=344
