I've noticed that many USB card readers are non-functional under Linux; according to the kernel developers, it's because relatively few of them implement the USB removable block storage class (protocol) correctly. While there are lots of expensive card readers that work with Linux, finding cheap ones is challenging.
I think, however, that I've found the ultimate *cheap* USB card reader that works with Linux: the "Tech-1 USB 2.0 CARD READER" at Dollarama. IIRC, it cost $2.
You'll almost certainly want to get a USB A-A extension cable for $1 or $2 at the same time, since the card reader comes with a 1" cable...