I bought mine from wcrepairs.com. They accept Mastercard/Visa and apparently even bitcoin. Might not be the cheapest, but it is comparable to everyone else and I had no problems with them.
You don't want Sky3DS because it can't play homebrew. The Gateway is your only real option here. It doesn't do cheat codes, yet (afaik, never really looked into it) But you can use some homebrew and commercial ROMs.
Can anyone verify the md5? The one on the site seems to be missing a number when I try to verify. Just want to make sure it's a typo. When I check it is identical, except for an additional '1' on the end.
I don't care. I won't use a card that has a hard limit on the number of things I put on it. Fix that flaw in your design and you might have a customer. Until then, I'll wait for the Gateway update.