I assume you mean microsd that goes within the gateway cart.
Really any legitimate card that is class 4 or over will be fine. Avoid ebay unless its a known trusted seller.
The class ratings are write speed, and cards are always faster to read than write. Read speed is what really matters for running the games and class 4 cards are fast enough.
Having said that I use a 64GB Class 10/UHS-1 sandisk ultra microsd, the extra speed is nice for when transferring games over to it via your PC.
If you are getting a MicroSDXC card (64GB or larger), make sure you have a card reader that is compatible. Many cheaper MicroSDHC readers wont work with the newer SDXC cards.
If you are going to use a regular 3DS (not an XL), you may wish to purchase a 4GB or larger SD card (full size not micro, the one that goes directly into the 3ds) also.
The regular 3ds shipped with a 2GB SD card which is too small for using gateway emunand.
XLs ship with a 4GB SD card which is large enough.