Modchipdirect is the one my friend used for himself. He actually ordered mine and a friend's for us since we didn't have visa. He lives in Canada but we live in the US. They ship out of NYC I believe. Very reliable and I got mine in 2 days of order.
Oh you may not be able to use a prepaid visa. You technically pay another country, probably china. Prepaid visa do not go international as far as I remember. A friend had that problem too.
I second Modchipdirect, this is the place i used to order my Gateway 3DS back in the GW 9.2 release. Very fast, the package had tracking (if i remember correctly), they even leave an invoice of your order in the package as well. My GW card came in a bubble wrap inside another envelope, and looked like it was handled very correct.
I'd save your money. 9.2 has cfw, and downgrading from higher firmwares seems imminent.
Yeah there is downgrading which is going to be available, but from what i know you cant do everything GW does on CFW. I've heard cases of people on CFW not being able to install the MHX Patch to play online, and others couldnt even get the game to run at all.
Things might have changed regarding that, but GW is a nice investment either way imo.