The R4 DS is the worlds most popular flashcards. This Nintendo DS and Nintendo DSi devices can be bought at several webhops, but aren't always on stock due to heavy demand and shortage of parts. Luckily, their are some webshops where you can buy the R4 DS cards and R4i DS cards at this moment:

Places where you can buy the r4 ds

Gamesyeah

Shoptemp