heya - would strongly recommend getting a pair of batteries (samsung 25r's are great for the subox) and an external charger (nitecore d2, for example) - can get both batteries and the charger for under $30 shipped from illumn.com and you can be confident that they are authentic batteries (lots of rewraps and fakes in the market - and not necessarily safe)...
the reason I suggest an external charger is that it is a safer, more reliable way to charge your batteries - you can use the usb charger in a pinch but personally, i use an external charger almost exclusively (got my subox when they were first released and have had very good results with it) - the charging board in teh subox is okay, but not on par with an external - using an external charger will provide a more consistent, regulated charge to your batteries and likely extend their life/ charge cycles as well - keep in mind, you are NOT dealing with a cell phone battery here - these are high drain, high amp batteries - ultimately your mileage may vary, but all of the research and homework I have done (a significant portion through this forum) points firmly towards using an external charger as a significantly better option
cheers - and vape on!