The software that is needed to turn your Nintendo DS into an iPod isn't supported by Nintendo. That's why you will need to buy these things to bypass Nintendo's restrictions.
1. Flashcard
There are multiple flashcards out there that are sold online - some offering more functionalities than others. All Nintendo DS's can handle simple homebrew such as this so any will do. Before making your purchase keep this in mind, with the debut of the Nintendo DSi all of the flashcards used in the regular DS and the Lite version were blocked. Those companies have now made new versions of their card that can be used on the DSi. If you are not interested in buying a DSi, I suggest checking if there are any cheaper options that only work for the previous versions, as the DSi versions are usually more expensive. However, if you are on the edge on buying the DSi or not, take in mind that the DSi cards the websites are selling are also backwards compatible with the DS and the DS Lite. Here are a few options to help you begin your journey to turning your Nintendo DS into an iPod:
Acekard 2 - DSi Version , Regular Version
EZ-Flash - DSi Version
DSTT - Regular Version
M3 Real - Regular Version (My personal favorite)
iTouch - Regular Version
2. MicroSD Card
A MicroSD card is needed to store the software that is used to turn your Nintendo DS into an iPod. Each flashcard has a slot where a MicroSD card can be placed. Since the files needed are relatively small, a 2gb card is recommended.
Here is a very cheap microSD card. Keep in mind this is a bulk package, so no retail packaging is included. All you receive is the card and a card reader.
Sandisk 2GB MicroSD
3. iPod Clone Software
This project has been discontinued, but the website and download is still working. You can get the software here.
LMP-ng
The download will be a zip file and you will need something like Winrar to open it. Once opened, there will be an NDS file and a GBA file. You will only need the NDS file.