I've bought my R4 and a 2gig kingston from them.
They mail it quite fast if you pay COD(Cash on Delivery) which is good for those people who don't have a debit/credit card to use!
Modsupplier is the most handy for me because they allow Cash On Delivery, however, they only have like 5 DS Flash Carts on sale.
(If you're asking if they are "legitimate," then yes, they are.)