visit the official site of the flashcart your looking to buy and they usually have certified resellers listed somewhere on their site.
That being said there are also some stores that will ship internationally too.
here's a list of good stores to buy from:
0ShippingZone(Low Prices, free worldwide shipping via hong kong post, may take a couple weeks to get)
Gamezway(Low Prices, worldwide shipping, free for orders $75 or more, may also take a couple weeks to get)
SimplyMods(a little more expensive than the previous 2, worldwide shipping, US Flat Rate(3-4 days) $1.99, US Priority(2 - 3 days) $5.99 and First Class International Air(no specified delivery time, but I'm guessing no more than 10 days) is $5.99
ConsoleSource(More expensive than the previous 3, but good reliable dealer, US and Canada only, shipping is $4.95 flat rate, you have to pay return shipping for defective items.)
iShopVideoGame(Cheaper than simplymods and consolesource, worldwide shipping, could be free depending on where it's being shipped to)
GameYeeeah(About on par with the prices at simplymods, worldwide shipping, they claim free shipping but it could depend on your order total)
KoolDS(prices on par with console source, shipping can be pricey outside the US($14 to $20), worldwide shipping)
GameKool(Prices and shipping pretty much exactly the same as gamezway)
5thDimensionStore(Free International Shipping, Could take up to 2 weeks to receive the item once it's been shipped, cost is a little higher than simplymods and a little less than consolesource)
GamebyGame and DealExtreme are no longer carrying flash cards, at least for now.