I guess the easy answer is just to call your local NetApp rep. I'm sure they can get you a card.
The reboot is necessary probably for software/firmware changes to the onboard "card" and perhaps to initiate some driver subsystems. You are sort of adding a new card and for most any OS, that means attaching drivers. This is all just a guesstimate from the outside. I guess this is something that could change over time with the ONTap 8.x kernel assuming one really felt the need to develop on-board hardware that could be powered down. It's not a very common task, but sort of a pain when it has to be done.
Is this card currently in a system? if so you can get the NetApp part number with the sysconfig -ca command. Otherwise, you'll need to use Parts Finder (under Technical Assistance) to search for the card to get the part number.