If you have many ESP8266 modules to flash with your own firmware this may be time-consuming, but Wing Tang Wong’s ESP8266 Pogo Jig Programming Board could greatly streamline the process, as it just hold ESP-12F module in place using pogopins, so you can go through boards quite quickly.
You’ll just need to a Wemos D1 mini board without ESP-12F module to the programming board, and then place your ESP-12F module (or compatible) between the pogopin to program it through Wemos D1 mini’s micro USB port.
The board is not for sale (yet?), but the EAGLE design files can be found on Github, and it should not be difficult to find a company to manufacture a few if you need it.
Via OSH Park’s Blog.