There's technically two kinds. The more common
SD2Vita adapter that you can easily order on ebay, and the more specialized
PSVSD.
The difference is that the SD2Vita is used via an adapter that will work through the game card slot (upon installing a special driver):
And the PSVSD requires you to own a 3G Vita, open it up and physically replace the 3G modem with the PSVSD PCB. It, of course, also requires a driver:
The main advantage of the PSVSD over the SD2Vita, is that it allows you to use microSD cards without taking up the game card slot. Therefore allowing you to make backups to your microSD card on the same system, and free it up for other uses. Like, perhaps, having a dedicated SD2Vita for PSP games.