You should check Ebay or OEM battery manufacturers, but since this laptop still hasn't been released for that long then there's likely still very few OEM battery option available.
I only see like one on Ebay right now:
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.XGL552VW+battery.TR...It's probably better to wait for a few more months and I'm sure there will be higher mAh batteries available.
If you're asking if ASUS offer battery upgrades for their laptops, that's a definite no.