I found a seller with a decent price on a few Acer AS09D70 5,600mAh battery packs on ebay.

These appear to be new, old-stock, packs. The listing has conflicting information. The title states they are 5,600 mAh and 56wH, which doesn’t quite make sense, since the nominal capacity is 11.1v. The listing explicitly states 4,000 mAh capacity, but the pack markings in the listing look like 5,600 mAh.

Someone cracked one of these packs one of these packs open, and found six Samsung 18650-28As, which are 2,800 mAh cells, which make these  62Wh.

The seller has these for $16 with free shipping, which works out to $2.66/cell. That also works out to  $0.26/Wh which isn’t bad, though I’ve seen $12 packs with 6 2,600 mAh that are a bit better deal.

One caveat is that these cells are supposed to be charged to 4.3v. Typical Lithium Ion chargers charge to 4.2v. If you charge to 4.2v, you’ll give up a bit of capacity.