If you're using an o3DS, you'll need a microSD adapter.

Why? You can just buy a large SD card.

https://www.amazon.com/SanDisk-Extreme-128GB-Memory-SDSDXPA-128G-G46/dp/B00NP63Y4K/ref=pd_lpo_147_tr_t_3?ie=UTF8&psc=1&refRID=PTPP96JPQC299WMEG9MJ