The only 64gb cards ive seen are these "Sandisk 64GB Mobile Ultra microSDXC Memory Card, up to 30MB/s read speed" I just bought one for $175 from here so not cheap but I wanted more storage for my note. They will get cheaper once more 64gb cards get released but sandisk is the best brand for SD cards.
I have seen them online for $165 but I only buy sdcards from people I trust and these guys have a shop near my house
where I get most of my computer stuff.
Not sure where you live but this is from Australia so you may want to check if they ship OS. I would never trust private sellers for any SD cards as theres just way too many fakes around.
Here is a speed test I did on the card using win7x64 with USB 2 which could be why I don't get much over 20mb/s with any of my cards.
Going by this test it would be a class 6 but they dont have a class rating anywhere apart from 30mb/s read speed which I dont hit. Just transferring over my 32gb cards data to this and am getting a speed of just over 7MB/s so id rate it a c6