We sell a USB adapter that you can use to power the WiFi adapter from OpenSprinkler’s USB port:
http://rayshobby.net/cart/accessories/acc-usb/usb-fa-mb
you can also buy this from Amazon. But the flexible one that Oasiz37 linked to is more convenient.
The reason OpenSprinkler still uses the USB B female connector is that the enclosure is made to fit this type of connector. We could have switched to use microUSB but that would require a re-make of the enclosure. Keep in mind that OpenSprinkler originated from an Arduino-based sprinkler controller, and even now Arduino still uses the same USB connector.