If you are using a Flash Cart like CycloDS/R4/M3, you can purchase the EZ Flash 3-in-1. In addition to rumble, it lets you run the DS Browser and GBA ROMs. Note: The rumble will not function with retail carts.
For rumble with retail carts, you can purchase Nintendo's DS Rumble pak from many suppliers. There is even a DS Lite import version available from importers. I use the EWin DS Lite rumble pak for my retail carts. It has received impressive reviews when compared to the official rumble pak. It is rather inexpensive and available from many suppliers online.