Found these 22650's for around £4 +P&P from a US supplier:
LiMnNi Rechargeable 22650M Cell: 3.7V 2000 mAh, 2.0A rated 7.4Wh -- UN Approved (NDGR)
I note on page 2 you say the standard pack uses 35 to produce 26.9v @ 10.6Ah (5 in parallel for 10.6Ah) so if we added 3 in series (11.1v) x 5 in...