Keeppower 3100s, CalliesKustom 3100s, Redilast 3100s, International Outdoors 3100s, AW 3100s, EagleTac 3100s, Orbtronics 3100s and the Xtar 3100s are all based on the unprotected Panasonic NCR18650A cells, which are well regarded.
You can get the Keeppower, Orbtronics and ETs in the '2 for $25-$26 range. AWs, Redilast and Callies are closer to $20+ per cell, FWIW.
I went with EagleTac 3100s, but have some bare Pannie cells that I run in my Fenix PD32 and ZL SC-600. For the Klarus XT-30, which uses two 18650s, you definitely want to go with protected cells.
Chris