BennVenn's flashcarts are quite excellent. He's recently released a very cheap SD cart (for the value of cheap: it's way cheaper than an everdrive, though), and his cart readers/flashers, as well as his more traditional carts, are very good.
Unfortunately, he's in Austrailia, so shipments take forever. Even more unfortunately, he's usually out of stock, so you have to wait for things to come back into stock, and then race to grab them. And it's not the cheapest stuff in the world, either.
You'll have to buy reader/writer for the GBA flashcarts, or the non-sd GB flashcarts. There's nothing external needed for the SD carts, though: they even come with an SD.
