Looking for an r4 card?

I have done various searches but have not yet found clear answers so I decided to ask a community: I recently purchased a 3DS. I am searching for an r4 card that functions with it. I do not necessarily need it to play 3d games; the main important part is that it can play regular ds games from the downloaded roms when the cartridge is inserted into a 3DS So I need: 1) A safe website which can sell me one (other than Amazon). If there are specific retailers which sometimes sell r4 cards, I would also like to know 2) A specific such r4 card as I described (I am not too familiar with the r4 cards so I need a good one for this).