eMMC interface : Parallel vs SDIO
The eMMC specification specifies a data bus width of 1, 4 and 8 bits, with the 4 and 8 optional. The interface does not support the slower SPI mode available on SD cards.

The vast majority of eMMC devices have 8 bit capable interfaces.

The power-up default is 1 bit mode, so that the device will be compatible with older controllers.

The physical interface and packaging (at least the pinout) is defined by the specification.

The interface protocol is MMC.

Kingston, amongst others, sells eMMC devices.