The M3 team (who are also the same team behind the great G6 flashcart), have released M3 adapters for the NDS/GBA. These adapters first came in the form of using a CF card, and as they started to use smaller storage mediums (physical size), they learned and made smaller and smaller products. There has yet to be flush adapter from M3 - but this is made up for in great NDS compatability and suport. The new M3L (M3 Lite) will supposedly fit flush in your NDSL - so stay tuned!
Website - [M3 Adapter]
Where to buy - [Global Sell Net]
Software Updates - [M3 Downloads]
Newest Product - The newest product is the unreleased M3 Lite which will fit flush in the NDSL and will accept MicroSD cards as the storage medium. This is the last of the great adapters to be released, but it will be worth the wait