I would recommend Playmods. Very cheap compared to other places and my Gateway arrived within a week. I got a faulty red card and was worried about getting it replaced, but they were really quick to communicate and sent the replacement out the day after I emailed them, no questions and no fuss. They're out of stock at the moment, but if you can wait and order next week, you can be sure they'll deliver a good service.
Unfortunately I have already made an order with http://www.new3dscard.co.uk/