You can buy the reference design, or get an Arduino+USB host shield. You can get an Arduino anywhere, I'm not sure if their firmware will fit on a smaller AVR like the 328p. You can get the USB shield from Oleg's site directly (circuitsathome.com).
Sparkfun sells a cheaper version of Oleg's shield, although it seems to be out of stock right now: http://www.sparkfun.com/products/9947. Note that this shield doesn't work out of the box with the mega 1280/2560 boards because its SPI pins are located elsewhere compared to the smaller 168/328p Arduino boards like the Deicimila. You can do a hardware hack if you really want, but long story short, just get Oleg's shield.
edit: I've had a look at the code, and I can confirm this will work fine with either Oleg's shield or the Sparkfun shield, on a standard Arduino. So price-wise I'd say about $45 plus shipping from SF when they get their shields back in stock.
Sparkfun sells a cheaper version of Oleg's shield, although it seems to be out of stock right now: http://www.sparkfun.com/products/9947. Note that this shield doesn't work out of the box with the mega 1280/2560 boards because its SPI pins are located elsewhere compared to the smaller 168/328p Arduino boards like the Deicimila. You can do a hardware hack if you really want, but long story short, just get Oleg's shield.
edit: I've had a look at the code, and I can confirm this will work fine with either Oleg's shield or the Sparkfun shield, on a standard Arduino. So price-wise I'd say about $45 plus shipping from SF when they get their shields back in stock.