Those are basically backup cartridges. Basically they are carts that were made to store one game and one game only. They are very hard to find as they normally have to be bought in bulk in are used by Nintendo DS game counterfeiters. They can be used to store one game on it and act and behave like a real game cartridge for all intents and purposes. They are not truly flashcards.
Also they can be expensive depending on where you get them and what size you get. The cheaper ones tend to come with 32 meg memory. The more memory, the higher the price.