Firstly, you should know that downloading DS games in this way is illegal -- it is software piracy. However, there are legal uses for these cards such as downloading "homebrew" software -- which is free, unlicensed, and developed largely by hobbyists.
The DS Fire Card is a clone of the N-Card (NAND Card). There are also other clones of this brand of card including: K6, MK5, DS Linker, FCard, and Ultra N-Card.
There are lots of different cards available. Some have onboard storage like the DS Fire Card. Others use external storage. A card with onboard storage is going to cost more in itself, but will likely work out cheaper since you will not have to buy external storage or the means to read that storage.
As for the DS Fire Card, it is not an especially popular brand of card. As such, it might lack support that the more popular cards do.
For a card with onboard storage you should consider the G6DS Real. It's the most popular of this type and the best supported.
Cards that use external storage are more popular though. They also have the potential to have a larger capacity depending on what size storage you buy. Modern cards use Micro SD cards for storage. The most popular and easy to find is R4DS. The second most popular is the M3DS Real.
You can find reviews of these cards and more at
http://gbatemp.net/index.php?showforum=99&prun e_day=100&sort_by=Z-A&sort_key=last_post&topic filter=all&st=0
Whatever you buy, you'll need to be careful you're not buying a fake. Fakes will usually work, but have inferior build quality and are likely to break. Going for a popular card ensures that there is a community of people online who are using it. You should be able to find forums that have lists of trusted sellers known to be se