Sold by
Acer on eBay. Free shipping.
Can also install Ubuntu (Crouton quick switch or ChrUbuntu dual boot).
This is the 2GB one with 4-cell removable battery. Adding more memory is easy, and useful even past 4GB on this 32-bit Intel because of the way both Chrome OS and Ubuntu use memory.
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EDIT: Anticipating questions, since I'll shortly be away from my PC for a while, here are some Ubuntu and Acer links that may be of use:
Chrbuntu:
http://arstechnica.com/gadgets/2012/...c7-chromebook/
Crouton:
http://lifehacker.com/how-to-install...-ful-509039343
Comparison:
http://it-nthusiast.blogspot.com/201...routon-vs.html
Hardware upgrades:
http://normcf.net/~john/chromebook/A...okUpgrade.html