Scott-S
Kona Fan
There are a few different brands, I got mine from iflash.xyzooh, sd card conversion <cracks knuckles, interrogates Google>
The later ones (Gen 7) can only go to 120gb
But my old iPod video has a 256gb sd card in it.
You can also get SSD for them but in terms of battery life the SD card is far superior plus realisticly an SSD would be wasted in an old iPod