If it is even can not work on the DSL, then you need to do a format of the SD card, and download the latest firmware V1.56b to try again. If not can not be fixed, i am afraid to need to buy another working card then.
I just checked the ndsgamer.com. they have sold out all of their flashcarts. Anybody knows why? And i also found this one: dsflashcart.com looks like it is a USA based site as they said here