By far the most common plug size for RK3066-based tablets is 2.5 mm x 0.8 mm or 0.7 mm (outer/inner diamater, respectively). This size is often used for generic Bluetooth headsets, and they usually come with a USB to 2.5 x 0.8 charge lead.
You can buy these cables individually (on popular auction sites for example), but they're not that easy to find for a good price, but if you want one, I'd suggest buying the cheapest BT headset you can find instead. The cheapest one on the largest auction site right now is $4.27 with free shipping, and it includes the 2.5 mm charging cable. A lot of sellers charge more than that for just the cable.
I'm not including the links because I'm not looking to plug any particular sellers or advertise, but the auctions should be easy enough to find.
Edited by MikeSD, 01 February 2013 - 10:30 AM.