I am not sure what device you are using, or if you are using a flash cart or not. In most cases, even a good quality modern slower speed SD card would be good enough for all DS and 3DS systems.
I recommend a Sandisk Ultra since they are reliable, affordable, and have more than enough speed for even a 3DS. Sandisk 32GB SD and micro SD cards are cheaper than $10 on Amazon, and they are good enough for most users.