I bought one from some generic Chinese vendor on eBay. I don't use it often, but it's always worked as advertised. I have an 8G thumb drive plugged into it right now.

$8 (including shipping!) from Hong Kong to the USA ain't too shabby!

If the link doesn't work just search for "n810 usb otg". "OTG" is "On-The-Go", which is a USB standard for devices like the Maemo which can operate as either a USB host or peripheral. The OTG cable signals the tablet to go into host mode without you needing to manually configure it in software.