Here´s one. Sonnet has a few more you can find at B&H. I only have USB 2.0 on my iMac, so thunderbolt is how I can improve transfer speed. The price is a turn-off though. I was hoping someone had found something a bit cheaper, that worked.
How about FireWire?
I don´t have firewire on my iMac or MacBook, so that is not an option for me.
Geez, $1K for a card reader??
How about a Belkin Thunderbolt Express Dock? For <$200, it would give you both FireWire and USB3 ports for a variety of accessories, including fast card readers.