There are several devices on eBay which state they will charge Kobo Voxs. Search on Kobo Vox Charger.
Note that many of the devices are for a cigarette lighter adapter which, unless you have the exact USB cable and/or resistor/whatever in place won't work. (Indeed one of them states 1000 mAh which, as we know, is insufficient. Previous sentence is wrong.) So these are quite misleading.
Two are the AC adapter type with permanently attached cable. One of which is 1000 mAmps and the Vox likes 2000 mAmps so I'd suggest the only one that states 2A or 2100mah.
http://www.ebay.ca/itm/5V-2A-High-Po...item43af643505 Note that eBay states they have sold 90 of them so they must work or there would've been returns. (Last sentence added.)
A third is the AC adapter with a USB port and an extra cable. I'd prefer that one as that device can be use for more than just devices with a micro USB end. However that device description is slightly ambiguous on whether it will charge the Vox so I've asked a clarifying question of the vendor.
Note that I have yet to try these so they may not work. Don't blame me, etc, etc.
Edits have been marked with explanations in brackets.